I see people in competitive telling me to give them the 2nd half and now I've seen it in pro play. Why do people fill it to half and then let a team mate defuse the 2nd half?

I recently got to silver. I HAVE NEVER PLAYED COMP EVERSINCE THE LAST COMP GAME I HAD. It's like everyone's playing alone. No one wants to plant. Everyone just lurks and goes out for kills. Players would get the spike and not plant, just go...