I wouldn't be so sure if that was the easy solution, as it depends on the cooperation of those companies.
They at least have the choice to resist in some way or another.
They also could be using both solutions simulatneously.
From their perspective, why not?
A lot of the communication won't be encrypted anyways, and some of it will be, but they may be able to decrypt it at some point in the future.
The hard solution isn't just a little bit harder ... it's several orders of magnitude harder and more expensive. It's also highly vulnerable to simply using encryption. The easy solution works because the US companies are bound by law to cooperate. There's no reason to believe that legal pressure on these companies has failed to get the government what it wants.
From their perspective, why not?
A lot of the communication won't be encrypted anyways, and some of it will be, but they may be able to decrypt it at some point in the future.