The issue, as mentioned a few time, is that in a USB-A dongle the electronics is almost entirely embedded in the USB-A plug, with only a small section of PCB (likely only the antenna) extending beyond the plug. Here is a teardown video of a Logitech unifying receiver:
There is lots of space inside a USB-A plug. There is no such space inside a USB-C plug, so all that electronics would have to extend beyond the plug so the dongle can’t be nearly flush to the laptop case. Any size benefit over a USB-A dongle plus adapter would be minimal.