You pay a lot to acquire customers up front but as long as your churn is net negative (ie., customers end up spending more year over year) you end up coming out way ahead. Sales reps only get paid on a contract one time, based on the value of that initial subscription revenue. $0 commission when it renews next year, and the year after, and the year after...

(Small caveat that someone probably gets commission on follow-on expansion revenue, but not that initial subscription amount.)