Then all links in all other files would need to be updated too.
Automatic backup system would consider all these files as new version, taking huge amount of disc space after each change of any title.
Also files might be referenced from outside of system,
Links stored in other systems/applications, external harddrives, internet, … would be broken.
On some file systems, titles added to filenames would exceed maximum length of a pathname.
Some characters are not permitted in filenames.
related: Show [first header found in a file] next-to an ID-only-filename
Edit: Also, Large permanent knowledge base should not have to be reformulated each time we want to use some little tool (e.g. Obsidian) on it. “unparalleled interoperability” is one of benefits claimed on Obsidian main page.