It's really not.
The problem is that you either need to be able to draw schematics or you need to be able to design in Verilog or VHDL. That means 99.99% of all programmers are excluded.
And this is before we start talking about the FPGA development tools which all suuuuuuuck.
If, however, you have something like the PRU on the Beaglebone Black (or the much more limited PIO on the RP2020 for example), your standard systems programmers can develop code for your board. That's a huge win over using an FPGA.
This, in fact, where I was really hoping RISC-V would shine. Have a couple of high power RV64I cores (Cortex-A equivalent) that can run an OS in soft real time and a couple of low power RV32I cores (Cortex-M equivalent) connected directly to the pins and running in hard real time. And, since everything is all RISC-V, the tooling is the same so you don't have to learn some esoteric thing (TI PRU or RP2040 PIO).
Alas. The problem is that the RISC-V people all want to be making the sexy thing instead of the thing that will ship a zillion units.
(I mean, that Microchip SoC is like $40 in quantity! You can get a pretty powerful ARM SoC for $20 and a remarkably big FPGA for another $20 and be WAY ahead.)