:-D
Why do 2000 players only seem to ever play bullet chess on here?

Mainly because when you play 15 10 you play cheaters. Blitz is also popular, there are less cheaters in blitzs at my level then there are cheaters in standard (unless perhaps the standard game is 90 30 or something like that). Also bullet and blitz don't take up a lot of your time. Those are a few reasons.

There are an annoying number of cheaters on the site, particularly with longer time controls (15 minute and above is a joke on most sites, but they also definitely exist at the upper end in blitz/bullet). But also, stronger players aren't coming here to hone their chess skills really, it's about having fun and shorter time controls are generally more fun.

Some, especially titled players, play games for money. For example you pay a coach for 5 (assessment) games. They are clever though, and they prefer to throw some moves without putting much effort - it's likely that they have good opening preparation and will get and convert a good advantage against the average amateur who doesn't know what he's doing), give you some adrenaline, make you believe they're great, knowledgeable and willing to pour their overwhelming wisdom into your head, take your five bucks and continue exploiting you as an impressed and not quite demanding student. All that in ~5-10 mintes.
Because in reality they are really suck players but they got touch screen apps on their iphones and they win on time because everybody else is fumbling around with a mouse.
Just curious lol