Fact of the matter is the SO pay rate is sufficient if they have enough applications to fill the roles.
Well that's not a true representation though. Take away the possibility of any promotion and let's see how well it goes then. It's a starting role in a big airline one which probably 95% of applicants are grossly overqualified and over experience for. So much so that people in the past 10 years if the timing is right have just been by-passing SO roles and sent straight to unpopular FO bases.