It used to be opt in until the icloud hacking saga where the public demanded something be done. So it was decided users want mandatory security by default. Almost all of these services provide backup codes you can write down on paper as well.
Sure, some people are going to lose their only device and the bit of paper, but at that point if you have literally nothing to identify yourself with, it's going to be hard to provide a secure service to you.
Sure, some people are going to lose their only device and the bit of paper, but at that point if you have literally nothing to identify yourself with, it's going to be hard to provide a secure service to you.