Setting to 100 is arbitrary. There's a large number of people that think it should be 80 chars (perhaps because that's how wide terminals were in the 80s). Others say 120. And so on. We could have a flag to specify it. But that would defeat the primary advantage of gofmt of producing one uniform style. There's no good answer to setting a limit.

It's much better to leave judgement to authors, programs, etc. that generate Go code, and respect the original input in that respect.

Will not change.