iPhone cameras aren't particularly great. They're good, but the thing that makes iPhone pictures really good is the iPhone's computational photography software. If webcams started including powerful CPUs, GPUs, AI chips, and OSs that can turn average pictures in to amazing pictures then webcams would be good. They'd also be quite a lot more expensive.
Except the webcam in my Macbook has a powerful CPU, GPU, AI chip, and an OS from the same company and its webcam still sucks.
Surely you could put all that software on the expensive PC?
A webcam upgrading from a 50 cent ISP (Image Signal Processor) to a $2.00 one and a $3 lens instead of a $2 one (numbers based on conjecture from the average selling price of a low-end USB webcam) doesn't mean you need to build in a battery, a cellular antenna, a Wi-Fi radio, a desktop-grade CPU and GPU, give it a sturdy all-glass design and add speakers to it.
Because the iPhone is a pretty expensive mass-produced camera with millions of dollars in development costs that a webcam would never have.
Even a highly-optimized “let’s steal a smartphone camera” webcam would never cost under $100, so it would sell extremely little.