There are baseband locking, so that the phone companies sell to carriers that pay for it and then give them to the users at subsidized price to make the money back in monthly fee. If they are easily unlocked, carriers won't pay too much for it.
Then there are different types of chips to support different networks. Even Qualcomm, the only carrier that can handle all bands and all networks, cannot fit the latest LTE chips with everything on it and has to split the solutions between CDMA legacy and GSM legacy to fit their LTE circuits. It may change a year or two later when they shrink the chip, but this may not be a big deal as many phone companies like Samsung have different version of their phones anyways.
Then there's whether the phone / chip support that AWS band in T Mobile or not. Even if the chip support it, if the phone hasn't gone through the calibration process in the factory, you may still not be able to use it on AWS after unlocking (think of your car's ECU swap with another car's ECU with a different engine size, even if your ECU are identical, the stored fuel maps are different).
Then there are different types of chips to support different networks. Even Qualcomm, the only carrier that can handle all bands and all networks, cannot fit the latest LTE chips with everything on it and has to split the solutions between CDMA legacy and GSM legacy to fit their LTE circuits. It may change a year or two later when they shrink the chip, but this may not be a big deal as many phone companies like Samsung have different version of their phones anyways.
Then there's whether the phone / chip support that AWS band in T Mobile or not. Even if the chip support it, if the phone hasn't gone through the calibration process in the factory, you may still not be able to use it on AWS after unlocking (think of your car's ECU swap with another car's ECU with a different engine size, even if your ECU are identical, the stored fuel maps are different).
