As a couple other users have said, the difference in cable lengths could be due to the difference in the way the devices are used as well as the difference in the devices themselves. In my opinion that would be the most logical conclusion if it's true that all MacBooks, even the cheapest ones, come with two meter cables, and all iPads and iPhones come with one meter. Remember, even though Apple advertises iPads as possible laptop replacements, they are tablets first. We know this because they don't come packaged with the Magic Keyboard. And I don't think Apple expects most people to use a tablet while plugged in. In contrast I think they do expect many people to use laptops plugged in because people often use them for longer periods and for more power hungry work, and more stationary than a tablet, and because laptops' larger batteries take more time to charge, so they need to be able to work while charging. So the difference in cable lengths makes sense to me.
Counterargument: first gen iPad Pro 12.9-inch came with a 2 meter cable.
iPad Pro has always been marketed as a productivity device with a detachable keyboard. What's changed since 2015? The only consistency is accessories included have become less and less.