Oracle uses port 1521. SQL Server uses port 1433. Why doesn't SQL Server also just use port 1521?
Specifically, I'm asking why SQL Server defaults to port 1433 and not another port.
You've got it the wrong way round.
1433 is the registered port for SQL Server.
The question should be, why does Oracle habitually use 1521
Mostly using standard ports makes it easier to configure firewalls.