Because they might need it (or at-least want it) for one stupid little thing, be that Facebook or their bank (and general computing defaults to phones these days for normies apparently, plus it's generally cheaper than buying a laptop).
My experience helping my Grandma begs to differ. Not to mention that Facebook and banking is what they're actually trying to do. That's the end-goal they're motivated to achieve. Random account setup bullshit and notifications about shit they don't care about is nothing but a hindrance placed in their path for nebulous reasons they don't understand.
But Facebook or banking is likely harder than the basic phone usage they're trying to get them through, including the same account setup issues.