While I don't have the background to answer why, the fact remains that without some basic maintenance, OS X 10.2.x gets slower with time. One cause is related to permissions and the other has to do with system updates. Again, I don't know the why - just that some simple maintenance can keep your system from being unnecessarily slowed in relation to them.

Issue #1 Disk Permissions. OS X is built on a Unix core where access to files is based on user level and permissions allowed. For whatever reason, these permissions seem to require a repair to keep them in proper synch. While having errors in the permissions can go practically un-noticed, as they accumulate, processes take longer and longer to complete - and in some cases bad permissions can even cause unexplained behavior of the system.

Recommended Weekly Maintenance:
Just run the 'Disk Utility' application located in the path: Your_HD/Applications/Utilities. Click on the 'First Aid' tab, select your hard drive, and then click on the 'repair permissions' button. You can only repair permissions on the OS X boot volume and that's also the only place that you need to worry about for this as well. Doing this once a week will keep permission problems from being a big system performance problem.

Issue #2 relates to the system updates and the apparent excess baggage they can leave. The solution to this excess update baggage is updating the pre-binding. While Apple says that OS 10.2 eliminated this issue (which was very visible under 10.1.x), I am not so sure. Either way, a very simple piece of software called 'Pacifist' takes care of this issue nicely. When you run this program, it offers a button 'Update Prebinding Information...' and you just click that button and it does the rest. For 10.2 and higher, Apple does state this shouldn't be necessary.... I know doing this doesn't hurt and on my personal system, I could see a noticeable difference in the CPU Monitor display while at idle. Before optimizing, there was a about 10 blocks of load on the CPU even idle right after startup... After running the update, the CPU Monitor reported just a single block of load, the minimum.

CPU Monitor is included with OS X by Apple. It is located in the path: Your_HD/Applications/Utilities - same place where you find Disk Utility.

Pacifist is a shareware application that is downloadable here:

This software actually does a lot more than just the prebinding optimization and it's a pretty nice shareware value at $20. Shareware is all about paying for something if you like it... And if you like a Freeware/Shareware app you are using, sending in that check helps insure its continued support and development + you're paying the author his due for the application you like you like to use. Neither OWC nor I personally receive compensation/benefit from user support of Pacifist.