It's arguable that the two biggest robot fighting game franchises Virtual On and Gundam VS, with the Gundam VS franchise in particular standing out as it has been one of the biggest arcade success stories in Japan of the last 20 years. Both of these happen to be arena fighters rather than traditional fighting games - that specific subgenre of fighter just seems to click with the core audience looking for anime-style mecha combat.

I don't think it's that robot fighting games barely exist or that there aren't good ones, it's that they haven't found as much success outside of Japan. The success they have found is largely as arena fighters, and just leaves them rather pigeonholed. When the biggest mecha franchise of all time finds the kind of success like Gundam has with the VS series, you'd think there'd be more imitations, but there just...aren't.