It's because some devs consistently release updates every month, and have never missed a deadline except to take a pre-planned one-month vacation once a year (which they clearly communicate with their patrons).
I get it, most devs aren't like this. Hreinn Games is definitely a poor communicator with a poor work ethic, which doesn't justify paying them. But don't forget there are devs out there who are excellent.
Also, don't look down on visual novels. Trying to write a good story, consistently, is extremely hard. Again, I don't think the writing in Hreinn Games is all that stellar, so I get where you're coming from, but some devs genuinely have good writing that is worth paying for.