Hedging bets prevents true intimacy
For the person in a relationship while still using dating apps (or at least not deleting dating apps), there's a simple, albeit highly problematic, rationalization.
Dating apps are overly saturated. And in this fast-paced, competitive meat market, a missed opportunity might never be available again. A lot of people feed into this by keeping their options open even after entering monogamous relationships.
One might think, "Well, if I don't check the dating apps, it's fine that they're still on my phone!" That's like saying, "Well, if I don't eat the rocky road ice cream, it's fine that it's still in my freezer!" I don’t know about you, but I don't have enough self-restraint not to eat the ice cream (though I'm more of a butter pecan gal myself) or check the dating apps if they're constantly pinging me every time I have a match.
Is that weak? No. It's human.
You're incapable of fully investing in the relationship you're now in when you keep part of yourself available to other people. Instead, you're constantly reassuring yourself other people are out there just in case -- and you might even be casting harsher judgment on the person you're with, looking for things to be wrong or not diving in to repair issues as they arise.