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.
I believe as more devices switch to USB-C PD charging, you will see more of the products you want on the market place. And technology will keep improving to reduce the size of the adapter.
I saw this on Amazon https://www.amazon.com/Satechi-Type-C-Travel-Charger-Adapter...
Something like this:
https://www.aliexpress.com/item/USB-Type-c-Wall-Socket-High-...
Or you mean an adapter like these:
http://www.tnpproducts.com/product/quick-charge-3-0-usb-wall...
https://www.aliexpress.com/item/Smart-Led-Display-Power-Adap...
but with multiple USB-C ports?