Because that makes you think you're paying less. That makes it more likely that you'll buy a license. And we like money - because 1) we like money 2) it gives us the ability to keep on developing on the product 3) it's a sign of appreciation from you to us. While nr1 is nice, nr2 is needed - but even more so- nr3 is needed too.
Edit 2020-11-12: We have now put the prices per year. It's just a little creepy to price it per month and bill per year.