Those who foil usually go out on a sb and tt as well. Hence no reason to sell. Foils just add more options that's all.
The other reason is boards last longer than kites. And while this should be the same for kites, people tend to hang onto what works well for them. While they do this with boards, once they found a suitable one. The temptation to try and get a kite which makes them boost higher, loop harder, drift more, pump up faster, go faster, slack more rad, have an endless wind range, look better, and work in 5 knots, is too big I suppose