There is an impedance mismatch between what is required out of filesystems in terms of backward compatibility and the features/capabilities that would be useful for applications like databases.

If you discard a requirement to look like a standard Linux filesystem to arbitrary applications, writing a purpose-built filesystem for a specific application often requires less code than trying to make off-the-shelf filesystems consistently deliver desired behavior and performance. And even then it is often difficult to guarantee that the off-the-shelf filesystem will really do what you expect in all cases. Installing a custom filesystem isn't always a practical option operationally but it is sometimes done for applications like high-scale data infrastructure because of the headaches it eliminates. I've built a few systems designed to be deployed either way.

I am not sure there is a "one size fits all" solution for this. Alternative filesystem implementations tend to make very application-specific and hardware-aware design choices.