Um ... because this is more fun?

Actually, it's because when I first started developing this system, I really didn't need anything as elaborate as Quicken -- it was just a duplicate of my checkbook register so that I could let Excel to the arithmetic on the balances and reconciliation. Then later I added a table for cash purchases (because I wanted to track what I was spending a little more closely). Then later I added tables for reconciling my credit cards. Then later I added budget categories to simplify my year-end financial review. And along the way I added all sorts of bells and whistles to automate regular tasks or to keep track of new types of information (like which month items appeared on a statement). So the system evolved along with what I was interested in tracking and recording.

But let's just go back to "because this is more fun". I use Excel and FileMaker a lot for both my home research projects and at work (well, not FileMaker at work, but similar programs), so it's useful to have relatively simple projects to practice my skills on in order to learn new features.