There is a number though. And some apps show the overall amount of upvotes and downvotes someone has. If there is a number that's enough for far too many people. There are literally games people play that are simpler than making the number go up with comments. "Number go up, me happy.". There is incentive.
Is there really no karma? You still get upvotes for your comments and posts and in theory you can sum them together to get the equivalent of "karma", right?
Unfortunately, it's not Reddit specific. It's all around the internet, everywhere.
New people won't check old posts. Old users may not spend as much time on the site, so they may post something that had been posted before. Even without any benefit, there will also be reposters who do it for virtual numbers.