Because many of us have come to absolutely despise the tedium involved in configurating the emulation and "frontend" software and getting everything to play nice with each other. I have been down this road with a MAME (+consoles) cabinet; thoroughly enjoyed every part of the process right up to the part where I had to configure Hyperspin, whereupon I went through a temporary period of hating life and cursing my decision to ever build the damn thing in the first place! Now I am happy to have it and everyone loves playing on it, but I would have gladly payed $$$$ for a turn-key solution that "just works" and suspect I really didn't save much cash at all in the end (especially if the value of my time and mental stability are taken into consideration).

Speaking for me, I love building things, working with electronics, and even writing code for PIC/Arduino projects, but when I am required to start mucking around at a command-line for hours on end, my eyes begin twitch and I start to contemplate how much better off I might be if only I were able find satisfaction in "normal" leisure activities like watching TV or getting drunk at the lake.

Also, I don't think anything other than the emulation software is "freeware". As far as I know, the ROMs and other assets are still under copyright protection; of course there is almost no chance that a hobbyist would ultimately get into much trouble, but legally we COULD be nailed to the wall if someone with a bit of power to do something were to decide to make an example out of us. To me, the fact (however unlikely) that things actually could be spun in a direction that results in me facing a judge (especially if I were to sell the game for a profit) is unsettling and takes some of the shine out of the finished product.