The objects are so bright and so numerous that they corrupt an entire imagers data for the entire exposure. In a CCD rows and columns can become saturated and hot pixels can spill over into neighboring pixels (blooming). When the imagers for the LSST were being designed, it was not anticipated that they would have to deal with tens of thousands of hot fast moving objects.

https://www.lsst.org/content/lsst-statement-regarding-increa...