Why is PostgreSQL data checksums not enabled by default?

According to user comments PostgreSQL data checksums have very minimal runtime overhead (both CPU and storage) but would allow (among other things) using pg_rewind for point in time recovery (PITR). However, data checksums are not enabled by default and enabling it on already existing HA cluster is not possible without pretty significant downtime. (If I've understood correctly, you cannot enable checksums on hot-standby only and promote it as new master once enabling the checksums were complete on the hot-standby.) Is there some poorly known issues if data checksums were enabled by default? Or is the default state (checksums disabled) just *due historical reasons* even though enabling data checksums would make much more sense in all cases?