It was tried, quite extensively, early on in the reprap movement. No-one managed to get it working reliably. The issue is that the pellets don’t feed consistently enough. This means the flow is inconsistent. This massively messes with the quality of the print.

There are theoretical ways to compensate. Unfortunately, most result in a huge jump in complexity and weight on the head. Neither is a good thing.

Basically. The benefits aren’t generally with the costs, outside of a few, very niche areas. It’s also now easier to source filament most places, compared to pellets. So even that isn’t a game changer.