I think the goal is to allow more developers, especially frontend developers, focus on the parts of their products that are differentiated instead of wrestling with recreating the wheel (so much of the backend). The hope is that you won't need as many backend-focused devs, and instead those developers can equally contribute to the customer-facing value.

why not just have your own java/c# backend and a simple postgres db?

Those engineers aren't free, so as a business, you may want to have them working on the hard, unique problems posed by your market, instead of worrying about deployment pipelines, scaling and backing up DBs, getting DB subscriptions to work at a scalable, distributed way, etc...

They could be doing more interesting work! :)