I think there are 3 reasons.
Time, to shoot both HFT and FT, would cause anyone in a relationship to suffer badly, so people have to choose one or the other, the second one they might try occasionally.
2 Guns, you can't do both with 1 gun, totally different set up, very expensive, and hard to justify.
Plinkers are plinkers, they bought their guns to get out of the house occasionally and have a plink, and they are happy doing just that.
Average Spring Gun Shooter