Why do guys lose interest so easily?

I’m asking for one of my best friends because she’d really like some non biased perspectives since most of our group doesn’t understand either. A few months ago during summer, she hooked up with one of the guys in our friend group, who’s also the ex of our other friend. No one seemed to have a problem with it including our friend since she’s dating a new guy now. They spent the whole summer together and he treated her like a girlfriend, taking her on dates, having dinner with her co-workers, etc. They also told each other I love you. We’re all in college and around 20-21.

But now I guess it all stopped. Once the school year started, he stopped calling her as much and only hangs out with us when she isn’t around. He’s even made out with other girls at parties we’ve been going to. She keeps wondering what happened and tbh so do I. He seemed like a nice guy and now it’s just awkward whenever we’re all together. I feel for her because this has definitely happened to me too and idk what I did. Why do guys act like they’re so in love with you but then never take it to the next level?