No doctor tells you to never go outside in normal situations. If there are high pollution or severe weather events they will tell you stay inside for the duration. I'm sure there are others, but they are not common so it doesn't matter that I can't think of them.

However many people only go outside in the context of walking from their car to the door of wherever they are going, and some who work from home can go for weeks without going outside at all. This is a bad thing, you need exercise (though it could be inside), but realistically many people are not getting enough.