MySQL's storage has some space limits defined in terms of bytes, and to express that in terms of characters it has to allow for the worst case.
If it allowed 255-byte space to be used as VARCHAR(255) in UTF-8, then an insert of 255-character long string with emojis would fail.
UTF-8 is variable length anywhere, not just inside mysql. If you don't want this behavior, you can either use UCS-2 or UTF-32. UTF-16 and UTF-8 are variable length encodings, period.
Most databases that have use char length for unicode actually work with UCS-2, and use 2 bytes per char, like MS SQL Server.
The question was why UTF-8 in MySQL has weird limits. It wasn't about what MySQL should have done in an alternative universe.