iPads/Kindles are better because they're smaller and lighter so you can position them with far greater flexibility.
It's smaller, it's lighter, it's by definition just way more flexible to use ergonomically. You can position it in lots of ways you can't position a larger 13" horizontal laptop.
When reading in bed I use a stand to hold my Kindle at the best position. I would need a much heavier stand to be able to do that with my iPad.