I can't tell you what motivates others, but I know what motivated me with GoAWK: intellectual curiosity, and the desire to learn how to use Go's profiling tools on a non-trivial project.
I suppose I'm jaded. I've been writing software for so long and have been so underpaid for it, maybe I'm envious of your ability to follow through on your intellectually curiosity with such flourish. Anyway, great work. Looks good!