I still play frequently, but I'm a relative newcomer to the game, having picked it up at a yard sale several years ago. I don't upload much because I'm insufferably lazy, if I can't hit an "attach" option on my post I don't bother; also, many of the parks I made a few years ago are loaded with custom content ...and I hate downloading anything with a long laundry list of other files I must track down in order to use it, so won't upload any such files either. All the parks I've made in the past couple of years are done solely with the default game since apparently custom scenery is not recognized by the game (it's eye candy for the player but has no bearing on park/ride ratings).
But RCT3 is nearly twenty years old, ancient by computer standards, and we have a generation of computer users indoctrinated to never install anything over a year old. I'm surprised, and grateful, this site still exists; so many other sites, for more well known games, have disappeared in the past few years. (RIP Blackhole Motorsports, NOLFGirl, Chieftain's Chambers, NOGrip, Hovercontrol....)
