Although it is possible to implement some popular algorithms of FPGA, there is a problem. It may require a custom and expensive PCB in order to match the GPU performance on existing algoritms.
Actually thats not exactly true FPGAs require not many external components and for mining you dont use all of the pins so less tracks. Back in the days when mining of bitcoin was possible on an FPGA most of the boards was 4 layers of copper and people managed to produce them quickly, for comparison a typical GPU board is 10-12 layers of copper which is out of scope for a lot of PCB manufacturers still, but anything that is 4 or 6 layers is cheap nowadays.
Also Monero is minable on FPGAs its highly probable that people are mining monero on FPGAs as we speak, of course i'm not a dev and cannot speak for the profitability of the FPGAs