Because he put in the same comment, that high street and highway are called that way because they where elevated over the other streets, which is nonsense.

In fact, high street/highway are that way, because in Old English high didn’t only denote elevation, but also a high status/rank/importance.

Modern English still uses that meaning, but it’s rarer nowadays. For example, high society, high sheriff or high priest aren’t called that because they are tall.

High is also used with a lot of words where elevation doesn’t matter: high rank, high value and so on.