Comment by ricardobeat

Because most mainstream frameworks are too bloated to handle more than a few hundred rps on a normal server, and caching has gone out of fashion (?), so it's the best way to save money.

All of this makes very little sense, but every couple years a guru will shout new commandments from his social media mountaintop and reshape the landscape. It's more of a religious ritual at this point, really.

On a more serious note, it's also because the market is saturated with developers who specialize on React, and this is what's easy and makes sense to reuse their knowledge and code (vs say, setting up a caching reverse proxy and dealing with invalidation).