The countries that do prepare for catastrophic megaquakes do so despite the fact that the incidence frequency is quite low, because the consequence magnitude is so great.

Even regions with few quakes may have a history of large quakes. The New Madrid region (southern Illinois, southeastern Missouri, southwestern Kentucky, northwestern Tennessee, northeastern Arkansas) doesn't experience especially frequent temblors, but when they do occur, they're doozies:

<https://en.wikipedia.org/wiki/1811%E2%80%931812_New_Madrid_e...>

The potential risks of PQC are large enough that preemptive countermeasures seem prudent.

Not entirely true. Australian emergency services routinely prep for earthquake despite having only one or two decent shakes causing mass damage in recorded post colonisation history.

That prep doesn't extend to building standards for higher magnitude quakes.

Appropriate prep is still important for a possible credible threat, even if it is likely to be a rare issue, and the magnitude of that prep differs by personal posture and exposure.