You cannot go to a AT&T store to buy a phone and then use it on tmobile’s network. That’s why there is a 60 day delay. For 60 days you pay AT&T for a service contract.
Why not? You're paying them for the device. Why should they have a right to stop you using it how you want? Imagine if buying a car also came with a requirement to only use the dealership's petrol stations for the first year.
You can go other places to buy unlocked phones.
I don't buy carrier-locked phones anymore, personally.
Something I learned from an AT&T store