Theres nothing that's fundementally stopping a manufacturer from doing that.

But it's designed as an internal connector:

-Full 16x slots are big. Bigger then any external connectors that I can think of off the top of my head, or 8x.

- your going to get an unpleasant surprise when you try to disconnect something while running without compatible hardware and without jumping through the hoops you gotta do on the software side to shutdown the power to a pcie device (and not accidentally power down something like your internal sata or USB controller). Same goes for connecting something.

- the devices are also in general going to be designed for internal use (exposed fans, exposed PCBs that you could shock with ESD, etc.).

- You can't expect users to know that e.g. plugging in a PCIe device might rerout pcie lanes from say your NVME controller to the new device and that will make buyers *very* unhappy and support tickets will go through the roof, your don't expect your VGA connector to stop working because you have used all your USB slots for example because to an inexperienced that's what it would look like.

- And i'm sure there's a lot more reasons I could think of that an external pcie connector is a terrible idea.