I think this is the first time I’ve seen an ELI5 on the front page where all the answers are wrong!

Yes, atmospheric nitrogen takes a lot of energy to convert, but plants already provide all of that energy to their bacterial and fungal friends in the soil. So that’s not the reason.

The real issue is that photosynthesis and nitrogen fixation are incompatible because of oxygen. Photosynthesis makes oxygen molecules and oxygen molecules destroy the machinery that turns atmospheric nitrogen into usable forms.

There are organisms that do both photosynthesis and nitrogen fixation, but they don’t do it in the same place or at the same time. Some photosynthetic bacteria (like Anabaena) make special cells called heterocysts that fix nitrogen but don’t photosynthesize, while others (like Cyanothece) photosynthesize during the day, then fix nitrogen during the night.

When photosynthesis and nitrogen fixation were first evolving, this incompatibility wasn’t an issue because there wasn’t molecular oxygen in the atmosphere to any appreciable degree. Photosynthesis created it all. Once it had, created an environment that made nitrogen fixation much more challenging.

As u/writingtherongs pointed out, plants COULD have evolved a separate compartment to fix nitrogen in, but that just hasn’t happened in the last ~3 billion years. They’ve always relied on other organisms to provide their fixed nitrogen.