Think of the hosting costs of a 30-minute video. If Imgflip can't afford having free one-minute GIFs, imagine 30-minute videos.
Also, Imgflip isn't YouTube. 30-minute videos aren't Imgflip's thing. You know what is Imgflip's thing? Meme GIFs (it's a meme-focused site). You don't need 30 minutes for Meme GIFs. You only need a few seconds. Videos would not only stray Imgflip too far away from its purpose, but it would also cause users to leave. Why would they watch videos on Imgflip, if they can watch videos on YouTube? This would cause a cycle where users would leave in favor of YouTube, as well as creators (since nobody sees their memes).
In other words, if Imgflip were to start focusing on actual videos, not only would they lose a lot of money, but also gain nothing in return.