In Germany there is a lowercase letter ß. It actually is a ligature of the letters s and z. It does not have an uppercase variant, because there is no word that begins with it. One word would be Straße. If you write that all in uppercase, it technically becomes STRASZE, although you almost always see STRASSE. But if you write that all in lowercase without substituting SS with ß, you are making a mistake. And although Switzerland is a german-speaking country, they have different spelling and rarely use ß -- if not ever.

This is just one of many cases, where case-insensitiy would give more trouble than it's worth. And others pointed out similar cases with the Turkish language in this post.

But the thing is that the file system doesn't need to be case-insensitive for your system to support human intuition! As others have said, people don't look at and use filesystems, they use programs that interface with the filesystem. You can absolutely have a case-sensitive system that nonetheless lets you search files in a case-insensitive manner, for example. After all, to make searches efficient, you might want to index your file structure, and while doing that, you might as well also have a normalised file name within the index you search against.

Now, as you said, UNIX did the choice that's easier for computers. And for computers, case-insensitive filesystems would be worse. There are things that are definitely strange about UNIX filesystems (who doesn't love linefeeds in file names!?), but case-sensitivity is not one of them.

I don't know if that's right. The most obvious way two characters can be the same is if they actually look exactly the same i.e. are homoglyphs https://en.wikipedia.org/wiki/Homoglyph

But no filesystem I am aware of is actually homoglyph insensitive.

Case insensitive filesystems picked one arbitrary form of intuition (and not even the .oat obvious one) in one language (English) and baked that into the OS at a somewhat deep level.

You say "human intuition" - are those using different writing systems nonhuman then?

Except that is not true, it is sometimes convenient, and sometimes very inconvenient and not wanted. My reasoning for file systems that are case sensitive is the following:

1. Some people want file systems to case sensitive. 2. Case sensitive is easier to implement. This is very much not a trivial thing. Case insensitivity only really makes sense for ASCII.

In the camp of wanting case insensitivity:

1. Some people want file systems to be case insensitive.

There is more in favor of case sensitivity.

But end users do not speak to filesystems.

Programs speak to filesystems.