I do that for some sites. It's the initial effort required to set everything up that's the obstacle. This kind of behavior is taking the easiest route at every step instead of investing time into making the future easier. I stopped using a password manager after it got hacked and haven't bothered setting up a new one.

The most annoying sites require you to confirm your throwaway password or log in again using it, and can't copy from the password field, so you can't just spam the keyboard and have to type it into notepad then copy and paste a few times before discarding it. This is stupidly inconvenient to do all the time but the alternative is more inconvenient to do once - researching which password manager is currently safe, finding it, installing it, writing down your master password somewhere really safe, etc. Then keeping on top of the news for the rest of your life to see if your password manager is going down the gurgler or been hacked. Also, will my passwords be available when I travel to a country with restricted internet? Who knows. Can I export my passwords to any other password manager or a text file if I need migrate? That's part of the research needed to even get started using a password manager.