People can see these with the naked eye (or with cameras). If our sophisticated defense systems are failing to match the naked eye... something is wrong.
My understanding is that only the balloon from last week was huge and easy to see with the human eye. I believe the other UAFs are much smaller. Also there were 3 or 4 during the Trump administration and noone noticed them with their naked eye
People think they notice lots of things with their naked eyes all the time and it doesn't make the news. Reports this week were that they didn't detect them in transit at the time, we only found out after the fact.
Another plausible theory is the Trump administration covered it up because of their deference to Xi in early 2020 (see: comments on COVID being under control).