What if your phone gets stolen? If you have a weak password then the person who has your phone would essentially be able to access any of your accounts.
What if you lose your phone? You would essentially be locked out of your accounts unless there was some way to disable it without knowing the 2FA code.
Both valid points and reasons why there is still a password in the mix, neither of them are a good reason not to use 2FA though.
If you did a very unscientific search on this board queering posts about people who have had their account hacked* I imagine you will find none of them had 2FA enabled, it is that good at protecting your goodies.
*
I hate using hacked in this context, I am old enough to still think of it in it's original context before any Angelina Jolie feel good pubescent teen movie.