And to the OP's point.
Separate content has been a thing in the past, but commercially it's not as successful as clips that feature feet and body tickling together, so commercial pressure has produced the results you see. Producers have to try and get the most out of a shoot so do both as it hits the most possible sales target.
In the 80's and early 90's you'd buy a video for $80 and 75% of it would be straight up crap. Stupid story set up (and I mean really stupid set ups with horrid acting and pizza delivery boy level dumbness), application of crummy bondage for 5 minutes, straight up bad tickling, or non responsive lees. The 'good parts' would be scattered through the whole thing, and amount to perhaps 10 minutes or so that one would actually care about. So you'd mash the fast forward button a lot. It was always a crap shoot as to if you'd feel you got close to your moneys worth and you'd not know til you got to shove the tape into the ol' VCR and see. With todays clips you basically can do the same thing. fast forward through the unwanted feet content and watch the part that gets you off. And you won't have as much mystery as if you'll like it. If the preview was enough to pull you, then it will probably work as you saw the lee getting tickled where you like to prompt your buy.
Would it be nice to get a 'pure clip' that only targets your area? Yup. But body part specific clips don't pull enough sales on their own to be profitable at the levels needed to overcome the piracy curve before they stop selling. So you get the whole body. And forced orgasms, and anything else that can get a few more sales before the content is slain.
Customs will be your ultimate answer if you truly 'need' a pure clip to enjoy. You find a lee you adore, hire her and a studio to make your 100% upper body clip and spend your few hundred dollars on it.
Myriads