I don't really know why, even though I did the same for a lot of years. That my laptop never left my desk is why, when it came time to replace it, I finally switched to a tower instead. If mobility isn't a concern, the cost/benefit of a tower is greatly superior.
Update: On consideration, the reason I kept buying laptops for so long was that I thought having the option to be mobile was valuable. It turns out, based on my actual usage history, it's not valuable to me at all.