You should see this from opposite side of view.
Instead of asking why validators having 100% max commission rate, you should ask validators
why some of them have less than 100% max commission rate.
The reason why some validators have less than 100% max commission rate is that they want to
ensure their delegators that, even though the commission rate is able to climb up, it is imppossible
to climb up higher than suggested max commission rate. Also small max change commission rate implies that such validators do not want to surprise their delegators with very fast commission rate hike.
Those validators who have 100% max commission rate and max change commission rate gives no comfort zone on delegators about how high or how fast it can change, while delegators might not notice the change for long time.