That's because the system is wrong. If the developer received the reward based on the updates it releases and not on leisure time (months in which they presumably work to release the game, obviously in the slowest way possible). They would be more productive and would actually earn for their work and not for lazing around.
But since Patreon is served by this nefarious system, it becomes a vicious circle of earning by lazing around.
True, I would even go a step further and say that it makes no sense, from a certain perspective, for the Developers there to really work all that hard. If you can get ~4k a months for not finishing your work instead of 4k for a few months, because you finished it and it is no longer funded -> it is almost illogical to ever finish your game. And that's where, imho, the true problem lies: To make a living from patreon the best thing a Developer can do is hype their game up, tease people with things you know they'd love to see in a game, make sure to raise your goals again & again to distract from the fact that you never really finished any of the starting goals, make the Patreons feel as if they are a part of the development, do whatever it takes to give them a sense of "something really big is in the making", but for god's sake, never finish anything. Cause only by waving around your idea below their noses will you make sure to generate a constant, in some cases ridiculously high, income.