Suppression of the populace is the only real reason.

They want to limit, even if secretly, the ability of the people to easily disseminate information and act on it by organizing protests or unifying themselves against an (unknowing) oppressive government.

If the government can fully read along with the chat channels and easily identify who is who and where they live, it can catch 90% of public anger in chats before it spreads amongst citizens in a wildfire.

For years now governments have spied on chats and open facebook messaging where they could, to stop or redirect fights or riots from being organized.
But they had to do it by directly planting someone inside the groupchats so they could read along, still it would not catch all chats between just two people.

Now with the growth of encryption(and related protective laws) that job is getting harder, so the way to control all this is to legally break open the encryptions and have an AI import, read, decide on, and forever store them for use as evidence years later. And an AI is not one single government worker but scales to be able to intercept and process billions of messages so its far easier to catch unwanted "behavior".

So it never was/is about "the children" or terrorism, these topics are precentage wise so low its insane to force strict regulations on everyone, these new laws are for what comes next, the opening of the doors to search for everything that the government decides is unwanted.

From being angry about forced immigration that comes with negative effects for the existing population, to public anger about an executive stealing people's pensions and getting off with just a fine.
From people organizing a riot to fight the opposing sportsteam fans just to get some excitement in, to people organizing blocking a polluting company's doors because they want to make a statement against unregulated pollution.

Every form of protest can be caught before it happens and redirected into manageable situations where the government gets its way or can prepare with a positive social media brainwash campaign or just introduce new laws.