Because in most languages, maintaining backwards compatibility is absolutely sacrosanct. It's not that you check the version number to know if your code will still compile. You check the name of the language to know if your code will still compile. (Yes, there are exceptions to this, but those tend to be cautionary tales. Python 3 is a better language than Python 2, but took a decade to gain adoption because it broke backwards compatibility.)

Since backwards compatibility is already a given for languages, you can then have the major version number indicate feature additions, rather than always being a constant value as semantic versioning would require.