The short answer is: because this is designed this way. A bit more serious: your "one would suffice" is nothing but your fantasy, related to lack of understanding how IP works. Your whole approach is wrong: you should first learn the basics of IP addressing (as the minimum) and only then do some conclusions and try to make statements.
Now, let's think: an IP address is a limited resource, because this is something unique, either world-unique, or unique to some network. Even though this is not a problem to have several IP address per computer, usually it has one or two, hardly more. And a single host, especially used as a server, usually executes several different services. Yes, on the same IP address. Hence, the port numbers. Another thing is this: IP address identifies the host, while port number identifies the "type of the service" and is associated with certain
protocol. Even though different port numbers can be used with the same protocol (thus allowing to execute more then one service accessible via the same protocol, which is an important feature of IP addressing), standard protocol have standard associated default port numbers meeting the world-wide standard supported by
IANA authority. By the way, any decisions related to port number should be made based on consulting this standard document:
http://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml[
^].
—SA