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.

Bitcoin allows you to transact with a system that works without relying on any others. You can be financially independent from banks to safeguard your money or inflation.

This comes with a huge responsibility, however. You become your own bank, and security failures in your computer setup (preventing either theft or loss) become entirely your own fault.

I don't expect that most people will make this choice, and instead will still rely on others. That is fine. The power Bitcoin gives is to at least have a choice to be independent, and hopefully allowing easier competition.