AIUI, the 8087 was essentially at the extreme cutting edge of what was possible to produce with the technology of the time, and even Intel at the time was largely treating it as a likely-to-fail project.
My educated guess is that primarily simply no-one has needed this, and secondarily it's hard. They're running softwares that can do all of their floating point in software anyway and they just don't need an 8087 on an FPGA. And floating point on an FPGA uses a lot of area, if one is taking the easy route of just emulating the external behaviour rather than the much harder task of emulating the clever microarchitecture that reduces it all to just 1 adder.