Comment by colejohnson66

The development side. Compiling and simulating your Verilog/VHDL can be done with open source software, but to put it on the FPGA itself, you generally need closed source (and sometimes paid) tools to generate the bitstreams. Contrast that with microcontrollers such as the ATMega which can be programmed from start to finish using an entirely FOSS stack - even the bootloader and programmer. And for some reason, these companies consider the bitstream formats trade secrets and refuse to document them at all.