Why do some men still flirt with taken women who clearly show they’re in a relationship?

TL;DR: I’ve been in a happy, committed long-distance relationship for over a year, and my social media makes it extremely obvious that I have a boyfriend. Yet I still have guys who know I’m taken openly flirting with me or expressing interest. I’m genuinely curious about the psychology behind pursuing someone who is clearly unavailable.

I’ve been with my boyfriend for over a year. We’re currently doing long distance between the West Coast and East Coast, but we make an effort to see each other every 1–2 months. Our relationship is good, and I’m not secretive about him whatsoever.
In my instagram, it would be basically impossible not to know I have a boyfriend. I have a highlight specifically for him, I’ve made posts with him, and I regularly put pictures of him/us on my stories.

Despite that, I have a couple of guys who still openly show interest in me, and I’m genuinely curious about the thought process behind it.

The first guy actually liked me for a few months before my boyfriend and I got together. His feelings apparently didn’t disappear once I became unavailable. Even after graduating college, he’s willing to drive about 2.5 hours just to spend time with me. (He asked he would like to see me for couple hours during weekend. Obviously i didn't say yes) He still tells me I’m beautiful and makes it pretty clear that he likes me.

Obviously I can’t read his mind, but I don’t really get the impression that his primary motivation is hooking up with me.
What confuses me is that if the situation were reversed, I wouldn’t behave that way. If I really liked a guy who had a girlfriend, sure, maybe I’d privately check his Instagram or Snapchat and still have feelings for him. But I wouldn’t actively express those feelings or flirt with him while knowing he was in a relationship.

The second guy is different. He gives me more sexual compliments, likes and replies to my selfie stories, and has told me that he regrets not expressing his interest in me in the past. With him, I get a much stronger impression that he’d sleep with me if I gave him the opportunity.

My question isn’t really about whether either of these guys likes me—I know they’re attracted to me in some capacity.
What I’m curious about is: Why do some men continue to openly pursue or flirt with a woman when they know she has a boyfriend, especially when she clearly displays the relationship publicly and seems happy in it?

Do they assume there’s always a chance she’ll reciprocate? Are they testing boundaries? Does being unavailable actually make someone more appealing? Or is it simply a matter of “I’m attracted to her, so her relationship is her responsibility, not mine”?
Or is it because they think they look better or can behave better than my bf?

I’d especially be interested in hearing from guys who have done this before. What was going through your head?