That is one of the basic rules but a lot of people use the same password on a lot of different sites. You would be surpised how much people even used the simplest passwords; I forgot where I read it but the statistics on top 10 of most used passwords are like: "123456" and "password".
The most commonly used passwords are the ones used by many people around the world for many sites.
This has made hackers to hack their accounts easily using a simple bruteforce.
We should prefer using different passwords for different sites but that could be a tedious process as remembering them can be hard.
Storing all these passwords on some device can also be a little risky but still better than other option.
Personally I have 2-3 set of passwords which I use on different sites to minimize the risk of hacks.