Actually, every FS fragments it just depends on how much and what kind of impact it has.
As a professor who specializes in kernel design and file systems, I could give you very low-level reasons why this occurs but I'm not that motivated and I don't know how many people would understand it.
To be brutally honest, HFS really sucks as a file system. When you compare it to some of the file systems available for Linux, HFS is very outdated.
Linus Torvalds had commented on how much he liked OS X but hated the file system because of its limited performance and scalability. You can Google for Linus's comments as well as others on HFS.
I purchased iDefrag from the folks at Coriolis and have been very pleased with the results. Also, the Coriolis staff have been excellent to work with. I also purchased iPartition from them as well. I do a great deal of development work on my MBP (lots of VMs) and my performance always takes a hit after a few months work.
I find if I run iDefrag over night I can measure the speed difference the next day. I would recommend it to anyone thinking of defragging their system.
Great Story:
I was at a conference where Linus Torvalds was speaking and somebody asked him about his public comment regarding how badly HFS sucks. He joked that his mailbox was filled with angry Mac users who demanded he retract his statement, offered proof that he was lying, and general fan-boy like threats. He said it was like arguing with people who think we didn't go to the moon...you know that kind of mentality from people who have no idea how file systems work.
Every time I see people make comments like the OP I think about this story and chuckle.
Cheers,
-P