Because people don't actually read what they are clicking on or even understand what they're doing. They just want to make the annoying banner go away. Same reason why people mash the next button when installing software.

Because companies are trying really hard to hide the "no" button: it's a single click to say "yes to all", but a safari through dialogues to say "no to all"

Same with websites like Youtube who don't understand a plain "no" but offer a fake choice between "yes, harvest all my data" and "ask me again later". That isn't consent, it's coercion.

They are choosing the lowest friction option.

1. accepting cookies is not the same as opting-in to advertisement

2. because most of the time, any other option is bloody inconvenient