And Trump let 3 pass, without any of them getting shot down.
This is not consistent with what's been reported. There were 2 previous balloons while Trump was still President, and one previous one after Biden took office. The Pentagon source stated neither the President nor the military knew about them at the time, and that they only found out about them 'after the fact.' Hence the thread title. The fact of the 3 previous balloons became known after Biden was already President, but apparently before the balloon that was shot down last week. Why they didn't detect them before, and how they figured out that previous balloons had flown over the US without being detected at the time, has not been reported.
That all sounds a little contrived to me, but according to the source from which this has been reported, Trump never knew about any balloons, and Biden didn't know about one that flew over the US before last week's that was shot down off the coast of the Carolinas.