Because it is not a cache. High entropy tables require a lot of vacuuming, which adds overhead to actual database operations. While you can set autovacuum par

Because it is not a cache. High entropy tables require a lot of vacuuming, which adds overhead to actual database operations. While you can set autovacuum parameters per table, they still are not free . If you need something like Redis or memcache, use that for anything that is heavy on writes and deletes.