Because the seller doesn’t want it to be and won’t proceed with the transaction if it is.

Imagine you’ve sold a car, the buyer drives off and later reverses the payment.

For very large transactions we have escrow—but isn’t quick by design.

There's no such thing as an unreversible transaction. There's merely transactions they don't want to reverse, which means you may have to go to court first, but you can still get the other side to send it back if you win.

You could lose the case or the other party could disappear. This happens in almost every fraud case, i.e. very frequently.

A new potential court-ordered opposite transaction is not what people think of when they hear reversible.

It's just a credit and a debit whenever it is. A reversal before it settles isn't actually much more real.

tell that to the victims of wire fraud. they wire their savings to the scammers number, and then can't get it back, or at least, not all of it. Even after a court case.

If the other end of the wire is a bank in the USA they certainly can get it back. It's only when the money has left the country that you're in real trouble

To be clear, it's safe to assume the money's left the country in instances of wire fraud.

When the money leaves the account, i.e. immediately in fraud cases.