While I do agree that it's a shame that there's little entertainment software that takes advantage of more "powerful" 68k Amigas I do understand why SDL and other less native api ports have become the main source of software for amiga based systems.
There's simply not enough coders left that know how to program specifically for the Amiga, not to mention it's more work to do so, and it's not easy to find good documentation to make amiga "native" software. SDL, Allegro, etc. however have a plethora of examples, tutorials and so on plastered all over the net. Things like gfx.lib, ahi, and so also often require more work due to the need to write a lot more own functions and whatnot vs. something like SDL (especially when it comes to games).
There's also the simple fact that more advanced games (stuff beyond stock a500/a600/a1200/etc) is a lot of work. If commercial companies from back in the day rarely made things that wouldnt today be called very retro, then its going to be hard for Joe Public to do it from his bedroom. On top of this it's *very* difficult to find people to work with that stick with things and maintain conviction. The sheer volume of work required to make a game to match the specs of a nicely upgraded amiga is either something beyond 1 person, or something that will take years.
All this said and done however, with the advent of things like minimig, natami, fpga arcade, and the fact that most people using 68k amigas have a higher base spec than yesteryear I still have hopes that maybe us 68k fans might still see a nice "amiga native" game or 2.
