Tight head is a more specialist position.
Equally, teams normally have 3 tightheads and 4 looseheads (with one who can swap). Or sometimes the otherway round.
All teams pretty much have 4 hookers. The lowest paid one will bring down the average of all the rest, far more than 3 number 10s or number 9s etc.
Locks will be well paid because whilst you'll only have 5 or so, 2 are starters and 1 is on the bench, meaning heavily weighted towards cash.
At least, that's one explanation.