Limitation is certainly not the exact reason I do chiptunes. I do chiptunes because I like the sound of the different machines I use and I like how you can compose extremely well composed songs with only 'chipsounds' instead of an entire song with different instruments like violin, piano, contrabass, and what not.
Although I like making music in general, making music with for example MusicMon on the Atari or Protracker on the Amiga is that you try to get a rich sound with all kinds of stuff happening in the background while only using 3 or 4 channels max. Not only because it sounds nice (if I'd only do it for the 'nice' I could also just make music in Modplug with 26 channels), but because it is challenging for myself to create a very 'wide' and nice sounding song while only using 3 channels. The limitation makes it more fun to create stuff.
On the contrary, I do like making music without the limitation because almost all of my/our (xyce) .xm's are 10+ channels most of the time.
I make music because it is fun, and I like the sound, that is probably the biggest reason I do it and why I love it. 
) !