Because it’d be horribly confusing.
An example from Sheffield, since that’s where it all started. A driver approaching from the south with a terminating or reversing train can take any main route except that into 2C (unless they’ve got a train short enough to fit). That’s 1, T, 2, 5, 7 or 8 route indications but not D. A driver of a through train can take 1, T, 2 or 5 but not D, 7 or 8. The driver has to make the decision quickly, often sighting the route indication from a fair distance, without reference to any written material. Can you imagine having to make that decision if you’re presented with a random selection of letters and numbers that bear no resemblance to reality?