Along the same lines, why spend all that time on this game when you are a programmer in real life! I look at games as a change of pace / relaxing not more programming. ;-)
In the case of TIS-100, I don't think the challenge of the game is 'write assembly' as much as it is 'deal with the somewhat crazy architecture and its self-imposed restrictions'. I've programmed some 6502 and x86 assembly in the past, but it didn't look anything like a TIS-100 program, which is more like a ridulously simplified transputer network with memory-less nodes.
for the same reason that airline pilots build their own experimental aircraft, long-haul drivers jump on a Harley and cruise around town, and machinists who make neat things at their home workshop -- freedom, excitement, the joy of exploration, the joy of working with the stripped-down essence of their expertise. TIS-100 really is different enough to be challenging and enjoyable, plus there's a neat story within.
I can play the real guitar but I still enjoy Guitar Hero.