Perhaps they do as some CC/debit companies/banks do and don't pay devs until a certain period has ended(quarterly/monthly/etc)?
It'd be very petty for a developer to whine about that since they had to know how it works as they were going in.
I don't believe this is it. And why would it be a problem now, all of a sudden, after the game's been on the store for years?
I wouldn't be surprised if it were more like having to beg GOG for someone to manually do the payment.
The same way we have to beg GOG to update the offline installers.
Smaller devs might need the money more quickly, and they might have been locked in a contract for x years.
You could also be right, though, but i'd assume gog wouldn't just manually cut checks and would have it setup to be more automated.....but then again, knowing gog.....