No good reason?
Loads of professional cameras still store on SD cards, and transferring the card over to the machine for editing is almost always going to be faster than just about any other connection method.
And HDMI is hardly "legacy". Show me a boardroom or classroom that has a Thunderbolt connection for its projector or wall display, and I'll gladly throw my dongles out the window. I haven't encountered a single one yet.
“things are this way so we should just support this way and not change”
no thanks. I don’t use anything but USB-C, for example, with my digital tools. Why take the SD card out of my camera when I can just plug it in via USB-C? Oh and charge my laptop.
why have all these dongles and ports and stuff? Big waste.