Why does he do it - dating apps?

I read stats that many men on dating apps are already involved with someone. Studies show up to 65% of men. My question is this. A guy (in late 30s) puts “Long term relationship” on a dating app section of what he’s looking for. The guy already has a LDR gf. Why not put “Fun, casual dates”? Why go on dates where you do not hook up? Why talk to girls who date only to marry? Also, should I try to find his LDR gf and tell her?