I think a lot of it is because it creeps up rather slowly, and you may not realize what’s going on with you until it’s too late. Part of this is because apparently when you’ve got early-mid stage dementia, your brain fills in the gaps for you in an attempt to make sense of things, and you don’t really realize that what you’re perceiving as reality is actually incorrect.
An example might be that a person with dementia may misplace their wallet, and then because they remember that they always put their wallet in the top dresser drawer, and that since it’s not there, then the housekeeper must have stolen it. In reality, they just put it somewhere they didn’t remember and had no recollection whatsoever of the whole event.
My mother has dementia, and that sort of thing I describe above is unfortunately pretty common. She didn’t even realize she was having gaps in her memory for a long time, because her brain filled in the gaps. Now she’s realizing that she doesn’t remember things, and is getting sad/frustrated that more and more basic activities are getting away from her grasp- things like being aware of the passage of time, and so forth.
The problem is that when she had the capacity to make any sort of decision, she didn’t really realize she had it (and would argue that she didn’t), and now that she’s somewhat more aware, she really doesn’t have the capacity to make any decisions.
As far as the best place to land if you have dementia; I’d say somewhere that’s compassionate, familiar, and aware of how to care for someone with dementia. Home might sound good, but for the people handling the demented person’s affairs, that might be a LOT more work- they’ll have to do all the maintenance and upkeep, as well as dealing with whatever healthcare personnel involved, as well as possibly having to replace them regularly if the demented person is prone to accusing them of thievery because they forget where they put their stuff. A facility is a lot easier for the non-demented people, even if it’s marginally more familiar for the demented person.
Another thing to consider is that demented people often express a desire to be somewhere else, even when where they want to be is somewhere they were desperately unhappy. This is because they often want to actually be some other time, not a different geographical location. My mom often wants to go back to her house and live there, even though she was desperately lonely and unhappy living alone, and has many friends and a better quality of life in her retirement community. What she remembers about her house is that it was some sort of paradise with wonderful neighbors, which was absolutely not true- she’s remembering 20+ years of good neighbor events distilled into “it was great”, without any acknowledgement of the time frame that all those good memories occurred within.