FPGAs aren't that difficult to program. Waves / timing diagrams are trivial to understand, they can just be very tedious to read. The different execution model to CPUs also isn't very hard to understand IMO. Hardware people love to say that it is, but it really isn't.
The hardest bit is probably SystemVerilog - it's just such a terrible language for hardware design. Full of footguns and gotchas and weird limitations and undocumented or tool-dependent stuff that you have to just know (like what is synthesizable).
Honestly, FPGAs are easy, but HLS is a deadend. The entire industry doubled down on HLS because of perverse incentives that encourage wasting FPGA resources.
The goal of an FPGA vendor isn't to make FPGAs mainstream, no. Their goal is to sell you the biggest FPGA they can get away with even though from the customer's perspective you want to use the smallest FPGA possible to make the economics work.
The hardest bit is probably SystemVerilog - it's just such a terrible language for hardware design. Full of footguns and gotchas and weird limitations and undocumented or tool-dependent stuff that you have to just know (like what is synthesizable).
Approximately nobody uses Yosys.