In an era where it's possible to book a ticket for a West End show three months in advance, (or more) frankly,I don't see the problem here.
West End Shows sell out so there is a need for it, we can use online ticketing to buy tickets on the morning of the game and still get the seat of our choice, so there is no need to do so.
There would be nothing wrong with having the option to buy further down the line, but there's no real need for it and it wouldn't generate any significant income because most wouldn't bother using it.