This may partially be due to the maturity of the two: desktops and laptops are, of course, continuing to improve, but at a much slower rate, and you probably won't get a huge benefit out of an upgrade unless you are doing high-end stuff. Tablets are improving dramatically with every release.
I used to upgrade my main computer yearly, but now doing so every 3 years or so feels more than enough. Most people I know keep theirs for even longer.
On the other hand, my iPad 1 - less than 2 years older than the iPad 3 I now own - feels downright sluggish and old. Many apps don't work on it, and I get tons of crashes on it due to memory issues. I think that trend will probably continue for a few more years (I think phones are in a similar position).
I used to upgrade my main computer yearly, but now doing so every 3 years or so feels more than enough. Most people I know keep theirs for even longer.
On the other hand, my iPad 1 - less than 2 years older than the iPad 3 I now own - feels downright sluggish and old. Many apps don't work on it, and I get tons of crashes on it due to memory issues. I think that trend will probably continue for a few more years (I think phones are in a similar position).