The problem is one of information provenance. A search engine shows you stuff that other people have written. It might be right or it might be wrong. But you get to judge that yourself, because you have an idea of where the information came from.

With AI summaries they take that away from you and claim the provenance for themselves. Google is now giving you the answer and they hide where that answer came from. Therefore, you cannot properly evaluate the provenance of the information presented. It still might be right or wrong, but you can no longer properly evaluate it based on source.