A software emulator has to be able to execute a single PS2 instruction in the same amount of wall time as it'd take on the original hardware. With a regular multiplication that's fairly easy: x86 also has multiplication, so you can do a 1:1 translation and be fairly certain it's within your time budget. With a bugged multiplication you need to do a regular x86 multiplication, and wrap that in a few dozen other instructions to add the buggy behaviour to it. There's a pretty decent chance it's simply too expensive!
When you're writing an FPGA emulator you are able to recreate the buggy multiplication directly in hardware. There's no additional wrapping needed, so (beyond figuring out intended behaviour) it's not any more costly than emulating a non-buggy multiplication. It's far easier to do a cycle-accurate emulation because you have direct control over the transistors!