By telling everyone they hacked something Using their real IP | Computer hardware and networking

By telling everyone they hacked something
Using their real IP address (Which is very stupid and only script kiddies do that)
Leaving behind a trace
Not using strong enough anonymity software
If they know that they had used weak anonymity software than not wiping everything on their hard drive.
Selling the stolen goods with their real name
Not deleting the logs
Leaving behind a backdoor that could lead back to you
And a lot more I can’t remember, hackers are humans they make mistakes but some sophisticated attacks are too well planned, they could be planning for months maybe years to counter any defense they would find on the target. But the defenders always keeps an eye on every suspect, one mistake could take the hacker to jail, google how many drug site owners from the deep web got caught even after they were using tor.