Filesystems aren't so simple. Modern advanced filesystems like btrfs, zfs and bcachefs are more than just filesystems.

E.g. they include features like volume management, compression and sometimes encryption. Most features can also be achieved with for example ext4 + lvm + luks, but it's nice to have all in one system with unified configuration.

tl;dr

Btrfs does more than ext4, which can have a negative performance impact, depending on the use case/metric. Usually the features gained by btrfs outweigh the small difference in performance imo.