Look how many C library functions implicitly call strlen() internally.

Also, look at functions like sscanf(). It can be orders of magnitude slower than fscanf() because every invocation calls strlen while fscanf incrementally reads from the current file pointer. I don't know why sscanf doesn't also work incrementally but the implementations I've tested don't do that.

The main point here being: if strings had a size_t size plus data, that would change an O(n) scan to an O(1) length lookup, and that would have huge performance gains throughout the C library, not to mention your own code as well.