My 4yo told me she loves me and I realized I forgot to say it back

We're leaving daycare, she's half asleep in the carseat, just goes "I love you Daddy" all drowsy and sweet. And my brain was somewhere else (thinking about work deadlines like an idiot) and I just said "thanks buddy" and kept driving. Didn't click until I was at a red light what I'd done. How do I forget to say it back?? I called her out at dinner and made sure to say it properly but damn that one stung. Parenting is wild.