cuz the mechanism for thrust vectoring can only do ~30 degrees of range before the thrust falls off dramastically, and it also has to respond fast enough for it to actually correspond to control inputs. versus VTOL is 90 degrees and slow like flaps.
you're assuming it's like a hand that can grab backwards but it's actually more like a penis on your hand so you can get an erection while you grab things. they're separate functions. spiders move their legs by getting erections with their interstitial fluid. see, I'm not completely retarded.
interestingly it looks like the nozzle is variable geometry so evidently the asshole puckering mechanism is a lot cheaper / lighter / more reliable than the one for thrust vectoring.