Why? I still legitimately really really like those games to this day, and yes I have played them recently. There's nothing out there that plays anything like them, at least in terms of variety of content. Like, I haven't played anything that plays similarly to the Knuckles stages especially, and the Chao Garden is of course a one-of-a-kind.
Well here are my reasons -
-Many Sonic fans seem to only want a game called SA3 but don't have a specific gameplay style or story in mind. (Evident by people referring to
Unleashed as SA3 then people arguing that actually Sonic 06 is SA3)
-Fixing the issues with the first two Sonic Adventures would result in a totally different game that doesn't need to bare the title SA3. Some like the treasure hunts, shooting galleries and frog fishing, but most agree that Sonic's levels are the best and would rather a game just about them.
-The story is finished and there are no loose ends. SA2 finishes in a way that could conclude the entire franchise and Heroes introduces a new story arc but that's resolved in Shadow the Hedgehog. (I could say Sonic doesn't even need a story and is better suited to minimal plots and the SA2 story is dumb as hell, but that's a more subjective viewpoint.)
-Current Sonic Team wouldn't be able to do it, I think. The best Sonic games are the ones where they focus on less gameplay styles and really put the work into making them shine. Lost World tried loads of different things and the result was a tonne of half-baked ideas that never quite reach their potential. Sonic Generations is a much more refined game because it takes two gameplay styles from previous games that people already like, and fleshes them out more. An SA3 would demand at least three gameplay styles and they could hardly lift them directly from SA1 and 2 because those camera angles just aren't passable in this day and age, and, again, many already don't like the other gameplay styles. They should just try refining one gameplay style rather than trying to do a bunch of shit that might not work, or we'll get another Lost World. Or worse, another Sonic 06.
-Negative stigma surrounding Sonic's Shitty Friends(tm) (I disagree here because I miss my boy Knux)
-Many people outside of the people who grew up with them don't find the Adventure games to be all that. They don't really have the unanimous praise that the Megadrive games have, so announcing it won't turn as many heads as announcing, say, Sonic 4. They should really make a Sonic 4, I don't know why they haven't yet
-They could always just put Chao Gardens in a game that doesn't carry the name SA3
I don't know why but I always end up writing a lot about Sonic when he's the topic of discussion. Apologies for the text wall but them's my reasons. And just to clarify, I'll always have a soft spot for SA1. I played it all the time as a kid and still find it a lot of fun now.