FTP support in a browser was always a nasty hack. It never supported most of the FTP protocol (or even basic file handling such as downloading a directory) and it only existed because HTTP transfers were garbage back in the 90s.
No, it existed because FTP servers have been around forever and have both better security and user/group-level access restrictions. HTTP has caught up in many aspects, but FTP's access is both simple and easy to administrate.
There's no excuse in 2020 (or at any time the past 2 decades) to serve a single file as a download from some website via FTP.
Except for the fact that you can point to an already extant FTP server rather than migrating all the files over to a web server and having to reset up logins, passwords, or whatever other access controls (if any) you feel you need.
Additionally, you're ignoring the fact that a lot of places still using FTP are often using them for reasons. Automated scripts is one of those reasons. Legacy applications and infrastructure often use a "if it's not broke, don't fix it" concept - and while sometimes that can be bad, in this case it typically comes down to a KISS reasoning, which is completely justified if you just need something to serve files.
Also, the big question is: What reasons are there for removing the functionality?
Saving space in the codebase? FTP is ancient, and the code for it is tiny.
Reducing need for code maintenance? Odds are it hasn't needed upgrades to the code in decades.
Better security? FTP's security is acceptable for most uses, and that's the sysadmin's call to make, not the browser's.