A better analogy would be, why is it so easy and quick to buy a 55gal drum of oil, but it takes an hour to change your own oil?

With an update, it isn’t just raw data being downloaded. If that were the case 150MB would beat 1GB every time. Instead you are downloading data and then doing something with that data (replacing files with new ones, possibly shutting down and restarting programs, making settings changes, etc). And that “doing something” is what takes time.