Let's suppose we accept your premise. Now, give us the details as to why that is not a good idea. Thanks in advance.
Generally, cops don't want to wait until the suspect is out of the house because they lose almost all control. You're introducing innocent bystanders into the equation. You have much less control over the background if rounds are fired. Would it be great if every suspect walked alone in an empty parking lot and you could just walk up and take him down? Sure, that would be great, but it rarely happens. Suspects go to schools and grocery stores and gas stations and bars and they mix with people who may also be carrying guns. Long story short, in operations like this, cops put themselves at a greater risk in favor of diminishing the risk to other people. That's the way it should be, IMHO.
If you approach a suspect at their house, you are most likely already familiar with the layout of the house, all the points of ingress/egress, their family members, their accomplices, and possible weapons in the house. You can cover the back and the sides of the dwelling and have ambulances standing by in the vicinity if needed. If you try to apprehend them outside the house, you lose those capabilities.
Were mistakes made in this instance? Yes. But the alternative could have been worse.