Also, if they're trying to do mass exploitation, we can try to deny them the scaling advantage.
After you spent 2 years of preparation and everyone has a backdoored sshd, and you've attacked N targets using it, the marginal cost of attacking N+1th target is way less than $5 for a wrench.
But if we can prevent these backdoors, it's no longer worth to spend 2 years on a backdoor that won't work, and instead they'll have to attack each target individually.