I'm guessing that in many cases, it's not one software developer who decides. Most people are told what to do, and for many websites I'm guessing that it's just some kind of Wordpress add-on.
Someone realised that they sold more add-ons if they implement those dark patterns, so they did it ("it's not me, I offer a good one but they buy the evil one"). In my experience in startups, the website was managed by marketing people who honestly had no clue: they seemed to genuinely believe that they needed those cookies ("I am in marketing, I need the data") and they did not understand the consequences. "I just install this Google thing, and then Google gives me nice data for free".
Why do people build weapons? That's a lot worse than a cookie popup, but I'm sure every single person in that industry will tell you that they "save lives".
Someone realised that they sold more add-ons if they implement those dark patterns, so they did it ("it's not me, I offer a good one but they buy the evil one"). In my experience in startups, the website was managed by marketing people who honestly had no clue: they seemed to genuinely believe that they needed those cookies ("I am in marketing, I need the data") and they did not understand the consequences. "I just install this Google thing, and then Google gives me nice data for free".
Why do people build weapons? That's a lot worse than a cookie popup, but I'm sure every single person in that industry will tell you that they "save lives".