In sane code, there are more function calls than there are functions. Imagine, now, that there's a function at 0x1337, and it's called from 69 different places in the code.

If we're using relative addresses, this would, of course, result in 69 different addresses to compress - each relative address being the difference between 0x1337 and the position of the code that calls it.

If we're using absolute addresses, we get the same exact address 0x1337 repeated 69 times - which is way more compressor friendly.