Why pin definition should be constant?

OK say compiler substitutes say code "LD, B(3)" which say means making PORTB.3 high each time I'm making HIGH BUZ because previously I had defined BUZ var PORTB.3

Now let's say, I want to do dynamic port re-mapping and at one moment BUZ is PORTB.3 and at another moment it is PORTB.4. What compiler should do? During the compile time, instead of replacing all references of BUZ with "LD,B(3)", it just places statement "LD,B(X)", where is X an address say in eeprom or ram, from which, during runtime, the number of port to be made high is being read. And when I need to force this statement to make say PORTB.4 high, I'll write 4 into that address, instead of 3.

I'm asking of something unusual and not previously done?