Unlike Windows based machines, MacBooks always use the AMD GPU when an external monitor is plugged in. I forget the exact reasoning, but it is the way Apple designed these machines.
In 2016 when they were redesigned I don't believe the intel IGPU could drive 4k @ 60hz, and Apple also wanted compatibility with the LG 5K display they partnered with. The best solution was to just force all external displays to be routed thru the AMD GPU.