

Token expiry. Because the same login token is used across many services, it expires at the same time. Of course, because the tabs are all open, they just know you need to log in again, not whether you’ve got other services open that also use your account.
A blockchain doesn’t actually provide control over a transaction, it just verifies that it happened. It’s supposed to be a long, immutable record of every transaction that has ever ocurred, which is precisely why transactions can’t be reversed. The work of verifying those transactions is performing by miners, or some other “proof of” design where you have an authority in the chain that confirms it.
Basically, it’s nothing more than a fancy receipt.