The issue is mold. Sealed plastic bags for old books is a recipe for mold. At least they're individually stored, so they would develop mold at independent rates. (And maybe the seal allows airflow.)

Most books have some mold by the time they're ~100 years old, it's just not enough to cause a serious problem. Sealed enclosures (wrappings, bags, tubs) are a nightmare situation. Even packing books too tighly on shelves accelerates mold growth to problematic levels.

If I did that I'd seal them with silica gel to keep the humidity down.

Dealing with mold in an archival environment is a nightmare, and yeah, the only thing sealing the books in plastic does is make it easier to either throw them away, or to freeze them to try to kill and then manually clean out the mold. https://www.nedcc.org/free-resources/preservation-leaflets/3...