We can (haha, farming), but it's much less efficient than synthesis (i.e. photovoltaics to produce electrolyzed hydrogen/methane which is then fed to hydrogenotrophic/methanotrophic bacteria that produce protein, starches, fats, etc), especially for protein production.

And it competes with nature for land and resources, especially water. And the need to keep pests at bay also causes big impacts. As does the run-off.