I'm no electrical engineer, but the spec says each usb-c port can provide up to 5amps at 20v, which you would need to do the full speed charge you wish for.

If you think about the size of the adapters that can do this already, you'd probably have to imagine nearly doubling the size for each additional port you added. Plus thermals, etc.

I could be wrong but I imagine its related to the physical limits of providing that much power in a small space.