It's not simply about clicking a button on Amazon, though, is it?
It's about remembering who/the age you're buying for, the card and the present and the wrapping, the date and time of the party, dressing child appropriately for the party/activity, travel arrangements there and back, ensuring the diary is free/juggling other arrangements...
Or for the optician/dentist it's remembering who needs to go when, any issues, diary clashes, transport.
They're all small things in isolation and no big deal, but when put together in a realistic package they take thought and consideration (that most of us do unconsciously). If you multiply that (because there's never just one thing to thing about, there are always many), that's a considerable amount of "mental load" and bandwidth used up, especially if you have more than one child or work.