Because all transactions, additions and rule changes are done by consensus of the miners.
And the miners are mostly automated servers that say yes to all transactions, yes to all additions followed by the guidelines set up at creation and no to any large rule changes. of which the changes are already quite limited by the already existing blockchain.
So much so that when a large theft has taken place, either a fork is created or nothing is done about the theft at all.
And thefts have been made.

This is why so many alt-coins keep popping up, as Bitcoin is easily surpassable in terms of function/technology and it’s also why “Satoshi Nakamoto” decided to quit the Bitcoin project and went on to create another.

Tether on the other hand is a promise that you will get US dollars for the Tether you buy. Their transactions are not done by miners, but by Tether Incorperated. Control Tether Incorperated and you control all Tether transactions.