Because there are people willing to pay it!!
I wouldn't.
4x4 have some attributes over 2wd, 2wd has some over 4wd, thye are a damn sight cgeaper to fix, can ruin more power through the gearbox as std, less to go wrong, as there is less, can replace 1,2,3 or 4 tyres at a time, more economical, quicker at the same power once under way than 4wd.
tabetha