There’s probably an anti-competition angle here: with end-to-end encryption, you need a trusted central authority to distribute the public keys, which would make interoperability across different messaging services more difficult (impossible?), something that regulators have been trying to make happen.
If I owned the largest messaging networks, I would enable end-to-end encryption by default too.
If I owned the largest messaging networks, I would enable end-to-end encryption by default too.