This 'maybe later' dark pattern seems to have gained so much traction that UX folk just use it because everyone uses it. There is probably some handwavy reason for it that amounts to not wanting to give the user the feeling they are doing something definitive, but it just ends up being another grain of sand on the micro-annoyance pile of today's tech.
> not wanting to give the user the feeling they are doing something definitive
In their refusal, while (accidental) acceptance is always definitive...
EDIT: In another thread this is called the ratchet of consent. Apt.