Everyone else did a good job of listing the rest so I figured it was implied so here goes.

Open carrying makes you a bigger target to hypothetical mass shooters because you are the biggest threat to them and will be targeted first before you can react.

It makes you a bigger target to hypothetical robbers who use your weapon to size up how much money you have/make.

It is clearly visible so the chance of social interaction goes up (see comic)

It puts guns as front and center as your whole personality. Showing anything as your whole personality is not great, but something controversial and divisive is worse.

And nearly all of this is avoidable by simply concealed carrying if you want to be armed.