There is something I just don't get in league

I watch a ton of LCS and spectator Challenger games and I've noticed that when teams are sieging a turret the opposing team will dance around until the turret goes down THEN they will engage on the sieging team. This makes no sense to me. Why not jump on them when you have the turret to retreat to if you still need to. This is mainly Inhibitor turrets. I understand not wanting to mess up and give up the inhib but if you are going to engage just do it.

Maybe someone way higher ELO than me understands their thought process a bit better and can explain it to me.