They probably don't want “safe haven for child porn” to be part of their brand distinction from other online firms.
Take this example where Pelosi, one of the richest members of Congress has personal calls with Tim Cook where she also holds her most worth stock of 17%:
https://greenwald.substack.com/p/nancy-and-paul-pelosi-makin...
It wouldn't be the first time... but it shouldn't be our first assumption.
Instead, in litigation caused by this scanning the companies of testified that they conduct the scanning out of their own free will with no coercion or incentive of the government, simply because they don't want their brand being connected to the distribution of child porn.
Implicitly, they'd rather be associated with unaccountable mass surveillance-- with products that violate human rights, and with a push of a button could be used to enable genocide.
They aren't stupid, so you have to assume that they think that the latter is less of a hit on their bottom line than the former. Lets prove them wrong.