They aren't headrests, they are safety equipment.
We tried out a 2017 Sienna and our two teenagers could not sit comfortably in the middle row. As fully grown, thicker adult, I can sit in the same seats without a problem. But if you are between 5' and 5'6" and thinner, your head lines up with where the head restraint sits, angled forward from the seat at about 30 degrees, so your head is pushed forward at an awkward angle. Great for reading a book in your lap, or bracing for a collision, but incredibly uncomfortable for any length of time.
Tellingly, one of the headrests (restraints, I know...) was turned backwards when we test drove it. You aren't supposed to do that or remove them because that will compromise passenger safety.
Toyota discussion groups talk about tilting the seat back or adding a foam back support, but that's ridiculous. Besides, driving with your seat reclined to any significant angle is also a safety hazard. In a collision, your lap belt is going to slide above your hips and crush your internal organs.