Yes indeed doesn't cost that much for 1 chapter but scanlation didn't stop on 1 chapter and 1 series. Besides if you suggest that readers should subscribed on official translation, that works only if every readers can pay dollars on those paid sites. This is why scanlations still existing and showing supports on artist and author by buying it on the sites where they entrust their work. Most scanlations put an announcement to support the artist or author's work by buying it once available on their country. Other thing is you can't say "why you don't just support the actually publishers" to the Patreons because it's their money and they are free on how and to whom they are going to spend it. Supporters are up for quality and context, there is why they gave back.