Why does it use custom tables instead of postmeta?

A site with a few thousand posts can carry hundreds of thousands of postmeta rows. Storing schema there means every lookup competes with the rest of WordPress for the same index, and it grows the table everything else depends on.

Purpose-built tables let us index exactly what we query — rules by target, schemas by type and status — and read a page's schema in one query. The check of every page keeps its results in a table of its own too, so it adds nothing to your posts either.