Because when you stop paying you have nothing.
I'd much prefer a hybrid model. Happy to pay $30 or so for a single version and then $1-$2/mo for updates. If I stop paying at least I have something.
The app Gameshow by Telestream works like this and I really like the model. It's $35 and you get one year of updates. After a year you either stick with whatever the last point release was or you pay another $35. I wish Adobe software was like this.