most PCs by that time had built-in MIDI synthesizers

Built-in? You had AdLib cards for FM synthesis, but they were never built-in and most PCs didn't even have them. Adlib cards used the Yamaha OPL2 or OPL3 chip.

Along came Creative Labs with their AWE32, a synthesizer card that used wavetable synthesis instead of FM

You are skipping a very important part here: cards that could output digital audio. The early Soundblaster cards were pioneers here (SB 1.0, SB 2.0, SB Pro, SB16). The SB16 for example was waaaaay more popular than the AWE32 ever was, even if it still used OPL3 based FM synth for music. It's the reason why most soundcards in the 90s were "Soundblaster compatible".

Digital audio meant that you could have recorded digital sound effects in games. So when you fired the shotgun in Doom to kill demons, it would play actual sound effects of shotgun blasts and demon grunts instead of bleeps or something synthesized and it was awesome. This was the gamechanger that made soundcards popular, not wavetable.

The wavetable cards I feel were more of a sideshow. They were interesting, and a nice upgrade, especially if you composed music. They never really took off though and they soon became obsolete as games switched from MIDI based audio to digital audio, for example Quake 1 already had its music on audio tracks on CD-ROM, making wavetable synthesis irrelevant.

BTW, I also feel like you are selling FM synthesis short. The OPL chips kinda sucked for plain MIDI, especially with the Windows drivers, and they were never good at reproducing instrument sounds but if you knew how to program them and treated the chip as its own instrument rather than a tool to emulate real world instruments, they were capable of producing beautiful electronic music with a very typical sound signature. You should check out some of the adlib trackers, like AdTrack2 for some examples. Many games also had beautiful FM synthesized soundtracks, and I often preferred it over the AWE32 wavetable version (e.g. Doom, Descent, Dune)