Why do some medical professionals and other caregivers talk to the elderly as though they are small children, even when their mental faculties are intact?

They’re not supposed to; at least there’s no written law for that. My guess is it’s a kind of method to create intimacy or gain trust. However, most older adults do not feel comfortable speaking to them with that tone or language. My mom’s cardiologist has this “annoying” habit, making her feel inadequate. But, people are different; some people may feel more comfortable with this method.