I could be wrong, but IIRC there is a limit on how much power can be delivered over USB-C that is arguably easy to hit for powerful laptops.

Perhaps that's one reason among others.

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Edit: Could the downvoter explain the downvote please?

Edit2: Quick googling tells me USB Type C supports upto 100W power delivery [1], while an example laptop, such as X1 Extreme Gen 2 can go up to 129W of power draw under load [2].

[1] https://en.wikipedia.org/wiki/USB_hardware#Power

[2] https://www.notebookcheck.net/Lenovo-ThinkPad-X1-Extreme-201...