Is being on a dating app cheating?

A dating app is meant for dating, not for making friends. So yes, if you are using it, you are cheating.

Why is he still on dating apps if he likes me?

He could also still be using dating apps, because – as great as you are – he doesnt quite know what he wants. Maybe he hasnt been single that long, maybe he isnt ready to settle down or maybe hes speaking to others to see if there is any interest there or if hes 100% into you.