It's not just pinch and zoom...
Tell you what - go to Newegg's mobile site sometime. It's one of the better mobile sites for buying stuff, but I still hate using it, so I don't. Besides the mobile site (most typical ones, anyway) often being very slow to load fully (even on 4G with full bars), having to scroll like crazy just to narrow the filtering options down is a raging PITA sometimes. It's a much, much faster and smoother experience on the laptop (even over Satellite Internet, FFS).
Many mobile sites are like this if the products they sell are even halfway as complex.
Sure, buying a T-shirt (choosing color, size, maybe long/short sleeve options) on the phone is drop-simple, but buying something with a lot of different options (like a laptop for instance) gets to be a bit of a rough experience, enough that I don't even bother unless I have a lot of time/battery to waste on it.
I don't blame Newegg (or similar) per se, because getting the UX right on a tiny screen is hard work as it is - it gets exponentially harder when the complexity of the purchase goes up. I do however blame the fact that a lot of this can be alleviated with a bit of cooperation by the carriers (seriously, Verizon - upgrade your shiz once in awhile and stop oversubscribing), and a bit of skull sweat on the part of the site designers.
Some ideas (aimed at no one in particular)?
* stop using so many fucking scripts/cookies/trackers/etc on the site - each one takes time to load, and mobile bandwidth often sucks. The longer it takes for the thing to load, the greater the chance your user will tell you to piss off and not make the purchase.
* clean up the interface (...more!)
* spend more than five minutes accounting for smaller screens. Not everyone has a tablet - sized phone, FFS.