In the ancient days, before the early 2000s there were no user and account management in webbrowsers at all. You started the browser and when you were done you closed it. If you wanted to hide your traces you could manually delete your history. Remember at that time this was on "the family computer". So your wife or mother used the same machine and could find your history.

This was the situation when incognito mode was introduced, if you used the internet at that time it was perfectly reasonable, and you knew what it meant.

It just happened that a generation grew up who didn't know how it worked and they assumed it's different.