Give it time guys. While it seems the alpha is apparently surprisingly playable and free of too many game breaking/crashing bugs, I'm sure they have had plenty of bug reports, some more serious than others and they are going to take a while to fix all of them. Plus I'm pretty sure there is still a lot of work to do on optimising/improving the building tools, their focus for the first phase seems to be on ensuring the game is stable and the modular building and pathing tools as well as peeps are user friendly and functional. Also consider that coaster building is technically not in the alpha, they clearly have a lot of fiddly work still to do beyond just adding more content!
Adding extra content at this point is just asking for more bugs to be potentially added into the build and more bug reports to be added to the end of the (probably) already long list.
This whole pre-release 'alpha' game thing has only really taken off in the last few years for better or worse. On the plus side it gives the developer a broader and quicker pool of testing data that they would have otherwise had to accrue from their own (paid) testers. On the other hand it means that expectant and often impatient customers are seeing the game in an unfinished state that would have in the past been months or years before public release.