I suspect it's because the number of people who install signal, briar, tor or v2ray is small, but the number of people who buy iPhones is large, and apple's defaults really matter.
If you take means to hide your activity, and you're in the minority, then that's a signal that you are suspicious. If everyone does it, dragnet surveillance stops working and it becomes harder to get information out of everyone, and the metadata of who is hiding their tracks vanishes.
I have a strong ideological objection to being spied upon and take measures to mitigate it. It's not easy though, and I'm probably in a minority. The government wants to keep it that way.
