If the hooked sail is pulled down via the downhaul the mast is going to present a nice curve, flattening the sail. If you have a halyard created at the bottom of the mast instead when you pull the downhaul the result will not be the same as the pressure on the front of the mast, where the descending halyard line is, will be getting on the way of the mast bending. Routing the halyard inside the mast track doesn't solve the issue either as now the under tension descending halyard need to follow the bend of the mast which is it going to do only by pressing on the sail rope. That's one reason we use hooks.

Now the second reason is compression. When you have a hook and pull on the downhaul the tension applies only on one side of the mast. If your halyard is cleated at the bottom of the mast the pressure applies up *and* down the mast, doubling the amount of compression the mast has to sustain for the same downhaul amount.

And a third reason is drag. Having the halyard hidden in the mast track reduce drag, and you can't put an under tension halyard inside the track.