Barely anyone uses FPGAs. They are pretty much only in use in highly specialized enterprise-grade hardware. Think a €5000 SSL accelerator.

For many FPGAs, the cost of the additional power supply controllers will be more than the cost of a full microcontroller solution.

Not all Chips are digital logic. In fact most are not. An FPGA is reprogrammable logic. You can not replace a rectifier or regulator with an FPGA.

That's kind of like suggesting someone use a stapler (not a staple) when they need a lag bolt because "well they're both steel".