It became possible to see them due to the effect that occurs during the transition from the light to the dark side of the Earth, due to which sunlight “illuminates” all objects in the Earth’s orbit.
All objects move along different trajectories and at different speeds. Moreover, UFOs sometimes fly on overlapping courses and fly in all directions and with different angles of the trajectory, at different speeds, and some stay in one place.
In 2020, Congress ordered the Pentagon to produce a report on UFOs. The nine-page report doesn’t say what the 144 sightings from 2004 to 2021 are, but does say that the government wants to collect better data.
You can watch this broadcast yourself in the video archive (time at 2.55).