Why is loyalty so underrated?

Like I had a guy ask me today that he fell in love with his co-worker as he's married.

Then he claims that he wouldn't cheat on her? WTF? I told him to leave his wife then do whatever he pleased to do so.

But why is loyalty so hard? I was loyal and I got cheated on twice and one of my exes tried to cheat on her husband with me. I don't get it.

Is it that hard to be loyal?