Because the characters are compelling.
They're complicated, but not in a desperate way. In most cases, these characters must make choices that aren't clearly right or wrong, like Daredevil or Luke Cage, and even Jessica Jones, who ends Kilgrave's life publicly.
These heroes are morally gray.
The Marvel Netflix heroes used to be normal people, and they're mostly surrounded by people who are not "super," and who the audience can relate to. In the Marvel Cinematic Universe, most characters are superheroes, and so are their friends. Within the Netflix Marvel world, the scope isn't as big, so the problems in the heroes' personal lives and relationships often take on greater importance. But that can also make the shows boring if the plot isn't tight.
While the Netflix Marvel shows frustrate me because I can’t remember them, I’ll keep watching because they’re well made, and I care deeply about these characters. Since the shows are a bit boring, they’re great background noise for when I’m doing something like cooking, laundry, doing a puzzle, reading, or just messing around on my phone.
But I am still optimistic about a day when one of these shows pulls me in to the point where it’s required, completely undistracted viewing.