Various hardware connectors that are dubbed "serial", e.g. USB and PCI-express, have many more than 2 pins/wires. In fact even the good old "serial port" has them.
But since they are serial, not parallel, one would think that they don't need so many. What are the extra wires used for, if not parallel communication channels?
The original serial had 3, 5 or even 7 pins, but only one to carry the data.
The three pins serial worked like this:
The extra pin for a common ground is needed so that the other computer knows what to compare the data signal with. The receive and transmit lines are crossed, so that data transmitted by computer 1 is received on the receive pin in computer 2.
If you want you might call this two serial lines.
The 5 and 7 pins versions of the serial cables added extra signalling, allowing to signal when they where ready to receive data, or when they where busy (if busy, any data sent might be lost since the receiver had no time for it).
USB (versions 1 and 2) have multiple cables, but only one pair to carry the data signal. Hence it is a serial connection.
Look at this picture (courtesy of Wikipedia). You will notice two power lines, and one data pair (basically one line with differential signaling)
PCIe has serial lanes. Each lane transmits its data in serial form. A PCI-e connection with one serial lane is referred to as PCI-e x1, one with four as PCI-e x4 etc etc.
This has two reasons:
If I signal data on multiple lines in parallel and the paths (cables+paths inside the device) are not of equal length then part of the signal might arrive out of sync. At high signalling speeds this was a serious problem.
Edit, as part of an answer to a comment. This is the pin-out for a PCI-e x1 connector. That one uses a signal lane on pins 16 and 17, but has many more connectors for other purposes (such as feeding power to the PCI-e card, signalling that a card is present etc etc)
Well, really the answer here is because that's the specification, but that's a bit of an oversimplification obviously. Let's talk about RS232 ("serial"), and that answer is sort of broadly applicable in the sense of "the manner in which these things are generally designed (at least at their most complex)".
In the case of RS232, you are correct - only two of those pins are actually for data transmission (one send, one receive). The others are for other relevant bits of the protocol that they've decided to give their own signal paths (detection, send and receive requests, etc).
To use the example of USB, it only has four pins - two data transmission pins, a +5V connection, and a ground. The USB specification rolls all the signalling that gets its own channel in RS232 onto the data channels.
(PCIe is a bit misleading - it's got two send and receive pins per lane...it's hard to call the overall design "serial" simply due to the lane architecture. Plus a bunch of grounds and power.)
For completeness here's the RS232 DB25 pinout used in early PCs
In my experience, most computers and serial devices only really used pins 1-8, 20, 22 at most.
Here's a typical diagnostic breakout-box of that era:
