There are compromises there. With page level compression the pages usually remain compressed in memory as well as on-disk and get unpacked on access (and repacked on change). This eats CPU time, but saves RAM as well as disk space which for some workloads is a beneficial trade-off as a larger common working set fits into the same amount of active memory (particularly when IO is slow, such as traditional spinning disks, cloud providers when you aren't paying a fortune, or when competing with a lot of IO or network contention on a busy LAN/SAN).
With filesystem compression you don't get the same CPU hit every time the page is read, and depending on your data you may get better compression for a couple of reasons, but you don't get the RAM saving.