I think you have that backwards. It takes a strong stomach to claim Google isn't facing competition on Google Sheets and threatening their market share would cause them to start caring. The more likely scenario is they have given up on winning any market share as serious users refuse to switch from Excel and non serious users have pads of paper and pencil, leaving Google just a tiny slice of the pie between those two groups to even fight for.

I would guess it has to do with what I have heard is the promotion path at Google. It seems you need to ship, and ship new things, if you want to be a serious contender. That leaves few engineers willing to take a look at Google Sheets issues and put together a PR to fix these things.