Because when you hear the other side (pretend to) hang up, you need to not be fooled and also hang up on your side. If you don't hang up on your side, then on older phones you actually have no way of telling the call is still going.
On your smartphone, it's easier to not fall in this trap, as the screen is making it harder to type a phone number while the call is still going.
No, this isn't just confusion, this is a real behaviour difference between smartphones and (some) landlines. If someone calls you on a landline, depending on the exchange, hanging up your phone might not be enough to end the call; when you pick up again, you may still be connected to the caller.
It used to be the case that the party who made the call - being the party who is paying for it - was solely responsible for terminating it; then timeouts were introduced; and on many landline exchanges there is still a timeout before the call is actually terminated if the callee hangs up but the caller does not - although these days it is just a few seconds, that may still be long enough that you stay connected to the original caller if you hang up then immediately try to place a call.
On your smartphone, it's easier to not fall in this trap, as the screen is making it harder to type a phone number while the call is still going.