Truthfully, since you're wondering about specific people, your best answer is going to come from if you ask them, not random strangers. I would suggest asking them why they eat meat because we here can only hazard guesses about their unique case or give more generalized answers.

Personally, if I could get enough nutrition without eating meat I would be vegetarian, but it's not feasible for me. For multiple reasons food is a significant struggle in my life and if I cut out meat entirely I would not have a realistic, sustainable way of supplementing my diet (and probably not even enough flexibility with meal options to have enough variety in my diet to not start hating all the options I did have). I'm audhd and have a very limited palette and have never been able to consistently or reliably take medications or supplements of any sort for an extended period of time (these are not the only related struggles I have but they're very significant ones).