But that is raw bandwidth. USB cannot transfer raw video because the host's GPU cannot output to USB.
That means the monitor most likely has its own its own video controller making it much slower.
Most laptops' graphics can barely handle gaming on the laptop's primary display so there is no point arguing about it being slower than the laptop's built-in GPU/IGP.
Most people who would carry an add-on display with their laptop would do so for productivity and presentations, not for 3D gaming. For video playback, USB3 has more than enough bandwidth to handle raw host-decoded video at 1920x1080p hi10 at 60fps so no problem there either.