Because Canadians don’t. 
Neither do we. At least, nobody I know does.
Er, you know America’s a big country with lots of different regional pronunciations, don’t you icephoenix?
I’m from Maryland, and yes, I would pronounce them the same way.
I don’t pronounce them the same; I’m from Ohio.
I’m from Maryland and I can orate the difference quite well.
I hope orate is the word that a good rider like me would choose. 
After repeatedly saying each word aloud, I’ll admit that the pronunciations are awfully similar. I can’t quite put my finger on the difference. The best way I can think of to describe it (keep in mind that I’m writing this at 4:15 AM) is that the D in rider is pronounced more, for lack of a better term, distinctly than the T in writer.
Clear as mud? 
Are you saying that some American dialects don’t have distinct sounds for “D” and “T”? Or are you saying that the words are both pronouned with short “I” sounds, rather than a longrer “I” in rider?
It just seems that if rider and writer are homophones in some dialects then either it’s a mispronunciation or else words like tight and tide and height and hide must also be homophones.
Yes I do, but I’m just posting what I’ve been told.
Thanks guys. I actually can’t imagine pronouncing the two words the same (I pronounce them differently, in the way Catalyst said). As for the people who DO pronounce them the same, I wonder if it sounds closer to my pronunciation or “rider” or “writer.”
RIDER: RYE-der (long first syll.)
WRITER: RUY-dur (equal length syll.)
Sorry I can’t find a better way of explaining it…
Sorry guys, in my previous post I meant to quote ruadh, not Catalyst. For some reason I can’t edit the post.
Pfft I’m tired.
RIDER: RYE-der (long first syll.) - “D”
WRITER: RUY-dur (equal length syll.) - “T”
No, because in those words the T and D sounds fall at the end rather than in the middle. That makes all the difference.
And it is a dialect difference and not a mispronunciation, just as the alternate (common in England) of saying the “t” in the middle as neither a D nor a T sound, but as a glottal stop, is a dialect difference and not a mispronunciation.
You may be interested in this past steel vs. still discussion.
I’m from Oklahoma, and I pronounce them the same way. I’ve been saying them over and over for the past few seconds, and I can’t tell a difference in the way I say them. Of course, if used in a sentence, a person can tell the difference by the context in which I use them. Most of the people I know pronounce them the same (middle class folks). And I work with some people from different parts of the country, and I think they pronounce them the same as well. But, then again, I haven’t paid a lot of attention.
Oh, and in regards to the “steel vs still”, I pronounce them differently…the way they look. Although in this case, I know a lot of people who pronounce them as “still”, my boyfriend, for example. He also pronounces oil kind of like “ul”. He grew up very rednecky.
And i just discovered he pronounces “wolf” like “woof”. That was a trip. I keep making fun of him for that one…heehee…
I live in Montana, but I’m from the Midwest (Illinois), and the Midwest accent is as close to a standard as American English has (CNN reporters use Midwestern English, regardless of where they’re from). While I probably don’t speak a `perfect’ version of the standardized version of the dialect, in this case I’m more than close enough.
rider: RYE-dur
writer: RYE-tur
There are two distinct consonant sounds, but otherwise they’re very close. I can easily imagine someone who’d pronounce them as homophones.
This Connecticutian pronounces them the same.
I often have wondered if the Connecticut accent is defined by the “glottal stop” as mentioned by ruadh. Note I’m refering to the common folk; not the ultra-WASPy characterizations heard in movies.
Most people I’ve asked in these parts pronounce the “D” sound in rider (and writer), but do not annunciate the “t” in curtain or mountain.
A lot of people around here pronounce them the same. Similarly, warder and water. In fact, I’ve had difficulty being understood in restaurants when I ask for water, but when I get my American-accented son to ask for warder, they understand.
I have always been under the impression that California English was considered standard American English. The Midwest encompasses a large area where there are many nonstandard accents, so it’s hard to imagine using it as the standard.
Welcome to the boards icephoenix. No one can edit the posts outside a mod or admin. This prevents people from claiming something and then going back and deleting the offending part of their post. Keeps everyone honest.
As to your OP, consider the difference between a NFLD accent and one from ONT. Right there, you’ve answered your question.
That’s a Southern American* accent. Up here in Montana, `water’ is pronounced as spelled. (WAH-tur)
(*Yes, that is what I intend to call it. We’re talking about Georgia, not Peru.)
So, do you see what we mean when we say that American English is just as fragmented into dialects as British English is? There’s really no such thing as an American accent, just various regional accents that all happen to occur in the USA. The Midwestern accent is what news organizations use, but the President is just as likely to speak with a Texan twang or a Georgian drawl, depending on where he’s from. America lacks a Received Pronunciation, in that there’s no accent taught in higher education as being correct' or proper.’ We muddle along just fine without one.