Because the cable itself is a factor. Just because the supplier says “I can give you 200 watts”, and the secondary device says “I can take 200 watts” doesn’t mean the cable between is able to carry that.

It’s not even just power but other things like DisplayPort over USB, Ethernet over USB, Thunderbolt, and whole slew of other things a theoretical USB-C cable can do, but not necessarily every USB-C cable.