Because it’s just plain wrong, that’s all. As Ferret Herder remarked above, vegetarians do not want well-meaning but ignorant people offering us seafood.
How about we consult dictionary definitions of vegetarian?
American Heritage Dictionary:
veg·e·tar·i·an·ism (věj′ǐ-târ**′**ē-ə-nǐz′əm) n. The practice of subsisting on a diet composed primarily or wholly of vegetables, grains, fruits, nuts, and seeds, with or without eggs and dairy products.
dictionary.reference.com:
veg·e·tar·i·an [vej-i-tair-ee-uhn] /ˌvɛdʒɪˈtɛəriən/
noun
- a person who does not eat or does not believe in eating meat, fish, fowl, or, in some cases, any food derived from animals, as eggs or cheese, but subsists on vegetables, fruits, nuts, grain, etc.
Wiktionary:
Noun
vegetarian (plural vegetarians)
A person who does not eat animal flesh, or, in some cases, animal products.
1897, Robert Hunter and Charles Morris, Universal Dictionary of the English Language, volume 4, page 5045:
Vegetarian Society […] A society […] formed at Manchester in 1847, to promote the use of cereals, pulse, and fruit, as articles of diet; and to induce habits of abstinence from fish, flesh, and fowl, as food.
So the cited definitions explicitly exclude fish. As for your claim that the definition of vegetarian includes fish-eating, it’s time to call for a cite.
Cite?
For a year Ive been following a JJ Virgin Diet which is:
no gluten
no sugar or sugar substitutes
no eggs
no soy
no peanuts
no corn
no dairy
I slimmed down, lost the dark undereye circles, lost the belly bulge and more important, I feel better now.
eta: after battling kidney stones last year Im off nuts, coffee and cola too, so mostly I eat lean meat, veggies, fish and poultry, and some fruits.
What I said is that vegetarianism used to include fish, but definitions change over time. In fact the vegans are pushing strongly for the ovo-lacto diet to not be considered true vegetarianism. Note for example your second cite:“any food derived from animals”
But if you want cites how about wiki:
"*Semi-vegetarian diets consist largely of vegetarian foods, but may include fish or poultry, or sometimes other meats, on an infrequent basis. Those with diets containing fish or poultry may define meat only as mammalian flesh and may identify with vegetarianism.[9][10] A pescetarian diet has been described as “fish but no other meat”.[11] The common use association between such diets and vegetarianism has led vegetarian groups such as the Vegetarian Society to state that diets containing these ingredients are not vegetarian, due to fish and birds being animals.[12] Individuals may label themselves “vegetarian” while practicing a semi-vegetarian diet,[10][28][29] as some dictionary definitions describe vegetarianism as including the consumption of fish, or only include mammalian flesh as part of their definition of meat,[9][30] while other definitions exclude fish and all animal flesh.[12] In other cases, individuals may describe themselves as “flexitarian”.[28][31] These diets may be followed by those who reduce animal flesh consumed as a way of transitioning to a complete vegetarian diet or for health, ethical, environmental, or other reasons. Semi-vegetarian diets include:
• pescetarianism, which includes fish and possibly other forms of seafood; "
*
Note that "The common use association between such diets and vegetarianism has led vegetarian groups such as the Vegetarian Society to state that diets containing these ingredients are not vegetarian," In other words, “vegetarian” is common defined and understood as including fish, but a group is lobbying hard to change that definition. I don’t think that word definitions should be based upon political pressure.
and
Top 7 Types of Vegetarians
1. Pescatarian (also spelled pescetarian)
The word “pescatarian” is occasionally used to describe those who abstain from eating all meat and animal flesh with the exception of fish. Although the word is not commonly used, more and more people are adopting this kind of diet, usually for health reasons or as a stepping stone to a fully vegetarian diet.
So, yes, the term used to include fish. Now, due to pressure from a special interest group, the term is changing it’s meaning.:rolleyes:
Now, I understand that if you call yourself a “vegetarian” but you don’t want dudes offering you fish. Ok. But some vegetarians don’t want people offering them any meat products. or Honey. or Milk. or Cheese. *Or cheese may be fine, but not cheese made with rennet. *
So, the term has so many meanings that the Op can’t possibly list each one “Ovo-lacto but no rennet” “Ovo-lacto”, “No milk or cheese- but honey OK” “ovo but not lacto”, “lacto-vegetarian”, “Jain”, “Yogic”, “macrobiotic” or even a common Buddhist definition of “meat is Ok as long as it wasn’t killed for me and I need food”.
To list every single variety of vegetarianism would be 'fruitless". 
Thus, the OP offered an umbrella definition for the purpose of a poll. Get over it.
FWIW, I call myself a vegetarian for the sake of convenience, but I’m actually a pescetarian. I have plans to completely eliminate fish from my diet once my bottle of fish oil capsules runs out (got some algae-derived DHA+EPA capsules to replace them), and aside from that fish oil and one egg-containing product I eat regularly I’m nearly vegan.
However, I don’t think “vegetarian” = “eats fish” is a correct definition. I have only done so personally because no one knows what the hell a pescetarian is, and I try to avoid fish anyway, so it’s easier to simply call myself a vegetarian when dealing with omnivores. I don’t expect people to keep up with the proliferation of diet terms and people certainly don’t need an entire spiel from me about what I’m perfectly willing to eat, what I try to avoid but will eat, and what I absolutely won’t eat. “Vegetarian” is close enough in the day-to-day. I’m already enough of a burden when dealing with work lunches and family meals without adding the additional hassle of precisely defining every aspect of my diet.
Dude, your *own cites *clearly distinguish between pescetarianism and true vegetarianism, and there is absolutely nothing there to support your claim that the term “vegetarian” used to be understood to include fish eaters. You’re just plain wrong about this, and Johanna is right.