The Mac Mini has it because one of the uses for them is in a data center where 10GbE is often found.
And I am guessing it is one of the differentiation marks between the iMac and iMac Pro.
If you need 10GbE on a TB3 iMac or MBP, you can add it via an external TB3 adapter. I am aware of models from Sonnet and AKiTi and I expect there are others.