Is it because they are used to scoring near the basket? Are their arms too long to get good arc on the shot? It's actually unusual to see a center who is a good foul shooter.
Here’s one of those guesses that leans toward the simple answer: Because, in the aggregate, among all players, the taller they are, the more likely it is that skills other than shooting are given greater importance when selecting them for a basketball team.
In other words, it’s not necessarily that in the aggregate players who are taller don’t shoot as well. But, instead, it’s that teams are more likely to select a taller person to play for them because of skills like rebounding, protecting the paint, and altering shots down low than because of their outside shot.