I think you misunderstand what USB-C is and what Thunderbolt 3 is. The one in Macbook's is not USB-C standard but Thunderbolt 3 using USB-C port.
The difference is the protocol used. The problem with USB-C protocol is that each manufacturer is allowed to not support all standard protocol for USB-C. Meaning, a laptop with USB-C protocol not (Thunderbolt 3) is not required to support all USB-C standard. For example there are laptops with USB-C port that are not Thunderbolt 3 and it does not support charging, video out or networking but it does support file transfer like USB-A. Thats why USB-C is broken in that concept because it's not like Thunderbolt 3 where once you use it, it is expected to support all those other features.
And I think thats the main reason why manufacturers are slow to adapt USB-C because there is no cost benefit in using the port. The problem is if supporting all USB-C feature is mandatory it will drive the cost for producing the port. For example a mid-tier phone using USB-C standard, if it is mandatory to support all features of USB-C it will drive the cost of the phone higher because those features needs different IC then drivers and etc.
Thats the reason why USB-C is not quite the port that will rule them all yet. For now most of USB-C ports (not Thunderbolt) functions the same with USB-A. Well, I have to admit USB-C connection is much stable compared to microUSB.
USB-C is the port to rule them all. The reason why all devices don’t have TB3 is because of cost. TB3 requires a controller for every 2 ports. But guess what? USB 4.0 will bring USB-C/TB3 to all very soon. Again Apple was ahead of the curve and this should increase the longevity of all of the TB3 Macs.