Because there is no clear indication to the user on the UI that the zoom level is greater or lower than 100%.
I noticed chrome is doing this indication by displaying a magnifying glass next to the bookmark star, but to be honest I just found this now doing this research. As a user, I could have had the same issue for months I would not know about it.
Maybe a solution is to have this in settings where you can turn this on until you turn it back off. And when you do it with ctrl+/- in the window the view will reset to 100% after you restart your browser.