Yes, you have been able to make your own parks since THPS2. But, they're small and limited compared to the courses the designers make on their workstations; especially as opposed to PC editing, where people are sometimes basically able to work with much of the same tools as the game designers (just not the source code).
I've seen some IMPRESSIVE user-made maps and such for Unreal Tournament, UT2003, and UT2004, as well as stuff for QUake, Half-Life and its mods, etc.
Hell, the whole mod/mapping/skinning/model making community wouldn't exist without the tools provided for, or available for when not included, for PC games.