Because real life isn't an action movie, and police would rather try to ram the door for a while and hope it budges, instead of instantly going for rappelling from the rooftop or for using a ladder. Especially given how relatively uncommon steel core doors in apartments are in practice.

One guy climbing through a shard-lined window is easy to mow down; the first cop that dies halfway through will block access to the rest. It also disarms the guy with the riot shield.

Opening the door lets a group of them quickly file in at once to swarm the occupants.