Why can't you just tell the blockchain that your bitcoin was stolen so that nobody can use it? They evangelize about The Blockchain. It cannot be changed or tampered with. It's TRUST without needing to actually trust any person. So, tell me. Why is it that when I watch Youtubes of how to set up a Bitcoin Wallet, I get dire warnings of: "You have to do it THIS way and take THESE precautions OR ELSE your Bitcoin might get STOLEN!" I mean, if the Blockchain is so pristine, why not just TELL IT that your bitcoin got stolen... and, therefore, no one can use it?
The short answer, is because "the blockchain" has no idea whether your bitcoin got stolen, or whether you actually intended to spend it. The blockchain sees a legitimate spend and a theft as the same thing: A transfer from one account to another.

What the setup instructions are trying to protect you from is losing the private key to your wallet. If somebody learns your wallet private key, then they can spend your money. The catch is, you don't know whether somebody else has a copy of your private key until your bitcoins are gone.