A common trait you’ll find among entrepreneurs is that they have tons of ideas and routinely can be described as ADHD. Due to this phenomenon, SaaS products, and the web in general, make for an ideal medium as it affords an elegant manner with which to constantly tinker. In many cases, you can have an idea, see it live in production, and get customer feedback in a matter of days or weeks (with strong automated testing, of course!). What other types of products allow you to do that? Physical goods? No. Services? Rarely.
Constant iteration and innovation really is adrenaline for enterpreneurs. It doesn’t get much better than SaaS/Web for products.