many have more antenna just because they have more than one wireless spec A/B/G/N/etc. Many of the newer ones have even more because there are certain things you can do to increase signal strength and reduce distortion.
One thing that comes to mind is Cell towers(you know, for your cell phones) use 3 signal sensors. There are calculations that they can do to remove echo from the signal that require at least 3. So, even if you had one large antenna with the same signal strength, you still would not be able to remove echo/etc from the signal. You need more antenna.
My router has 8 antenna internally.