It's not as cut and dry as you are stating. We will enter a structure that is showing no outward appearance of collapsing to assess the conditions inside, to help evacuate, find the injured, etc. Only then will a decision be made to abandon a structure.
The conditions on the affected floors in the towers was not immediately known. The calls coming into 911 from the towers
were not being relayed to the first responders. The firefighters had to try and get that info first hand. This is all discussed at length in the 911CR. I'm guessing you didn't read that, neither.
Once WTC2 collapsed, the evacuation of WTC1 was ordered...however, the
radio repeater system failed. Unfortunately, I fear that even if the word got to the firefighters to evacuate WTC1, many of them would not have left in time...whether it be that they had to descend 90+ floors in less than 30 minutes, or they would have taken their time to rescue as many civilians as possible.
They did not make the same mistakes with WTC7. Once everyone was evacuated, they set up a perimeter.
Only due to the lack of information as noted above. The firefighters never really got a chance to properly assess the conditions on the affected floors. It seems as most of the operations involved getting people away from those floors. Had 911 operators relayed key bits of info to the ICP, we may have not lost 343 fireman that day.