Because it really shouldn't be that hard to implement.
You can make symbolic link(junction i guess its called in windows) and do it that way, its not a big deal.
Also with current HDD prices I really dont understand why you just wouldnt spend $40 for a 500GB or $70 for a 1TB and just have a drive just for games. SSD's have been tested for games and dont show much improvement over mechanical drives gaming wise so why waste the space on your SSD for games anyways.
first off, you're grossly simplifying the situation strictly to level load times
even if we do that, if you ever play a game that has a lot of loading, even if a SSD is only a few seconds faster that compounds over the course of the gaming session
second, if you're ever playing and the game has to access the hard drive at all, get ready for some slide show stuttering with a disc drive.
third, SSDs can make a night and day improvement in gaming experience in certain, WoW for instance.
It really isn't that difficult to understand, I play a few games where I want as few hardware restrictions as possible and thus I want those games on my SSD. The rest of my games, mostly single player games I'll only play through ever so often, can go on a HDD where a hiccup here or there won't get annoying or ruin the overall experience because I'm only playing through it once and certainly not playing it competitively.
At the end of the day its pretty lazy/foolish of Valve not to offer such flexibility firsthand. Heck, they just recently introduced tabbed windows to steam chat, about 7-8 years behind the curve. Based on that we'll probably have multi TB SSD drives available for cheap and thus have no need for such a function.