That's not really true. The era of IBM designing computers was like the 8086 era. Clones were already dominant by the time the 286 came out, and Doom came out about half a year after the Pentium, or 586. That was very far into the era of "IBM compatible" machines being gaming machines.
There's more than a decade gap between "IBM compatible" clones taking over, and Doom coming out.
But even the clones, until the mid nineties, had at most a SuperVGA video card, which was little better than the annoying IBM VGA video card, which was certainly not designed for games.
Also, the clones had nothing better than the PC speaker for audio, even if that could be used very creatively by games like Monkey Island. The PC speaker was most definitely not designed for games.
The SoundBlaster cards can be said to have been designed for games, but they were a separate product that could be bought as an add-on for IBM PCs or clones.
Only during the second half of the nineties, after Windows 95, IBM PC clones with video cards and audio cards really suitable for games have become common.
The computer my family bought about 1-2 years before Doom came out (and which I eventually played Doom on) had Soundblaster, stereo speakers, SVGA, a CD-ROM drive. I feel like folks are mixing up eras a bit here.
Pentium arrived at the same time as Doom, but wasn't something you could reliably target as your minimal requirements until Quake, and even then it was controversial a bit.
The fact that they could run games was great for a lot of us. But they weren't designed to do it.