The official name is not "Chat Control" but rather "The Regulation to Prevent and Combat Child Sexual Abuse".

Of course, people are free to claim that it won't work, or that it's heavy handed, or that it's secretly nefarious, but there is a plausible answer to "why?" in the title itself.

Plausible? Think of the children is a stock lie that is never used in good faith. The children do not in fact thrive in a repressive dystopia.