1) You underestimate how critical prototyping has become, again likely since you say it's been a couple decades. Time to market has become more important, and verification has become harder as CPUs have gotten even more complex. FPGAs enable cosimulation and emulation, leading to faster iteration of both design and verification efforts and thus better TTM.
FPGAs are so important in the hardware development process that I would even say you're not a serious hardware company if you don't have any FPGA frameworks to design silicon.
2) As others have mentioned, FPGAs are also critical for low-latency workloads that require constant tweaks-- high frequency trading (ugh...) comes to mind. The need for "constant tweaks" could also be satisfied with just "normal" software, but that has higher latency as opposed to an FPGA, and FPGAs can get some crazy performance if you're willing to pay the price (south of 7 figures).
Overall sure, usage of FPGAs might be niche compared to, idk, Javascript; but it's commonplace/practically essential in hardware.