Nothing magical about a serial port. It is just an interface. IrDA, Blutooth, GPRS, Ethernet, ... are a short list of other serial interfaces. The physical interface and the communications protocol are two completely separate design decisions when constructing a device. Some serial interfaces have a big reason to always use a standardized protocol. You can't mix a random Blutooth headset with a random mobile phone if the standard doesn't specify both the carrier and the protocol. And for Ethernet, the network cards have hardware acceleration spefically for the IP protocol - besides the fact that without a protocol standard you wouldn't be able to use switches or hubs.

But look at a normal RS232 port. What you have is an electrical standard specifying voltages. And you have rules about timing with start and stop bits. And you have a mechanical standard so your connectors will fit. But there are no transport layer protocol standard. RS232 is just a raw channel where it is 100% up to the product owner (or customer) to specify what data to transfer and how to encode the data.

I own a label printer that can only print bitmaps, despite having a serial port. But that isn't really a limitation, since I can print anything I want with control to the last dot.