You would think that scientists would spend more effort trying to create "super" rhizobacteria that can colonize even more aggressively and also fix many times more nitrogen than ordinary ones

I think the problem with that idea is that the rhizobacteria needs energy to break the nitrogen tripple bond. A lot of energy. It gets that energy by getting carbohydrates from the host plant - which in turn are not available to the plant to grow.

I'm sure there's plants where not fertilizing with nitrogen at all would make the trade-off worth it, but the vast majority of our optimized crops can't spare double-digit percentage points of their total yield to feed bacteria. You're much better of just using natural gas (or green hydrogen) to fix the nitrogen.