When someone calls and I expect it to be 60 seconds or less, I don't wanna go fumbling in my bag for 30 seconds to find the headphones and spend another 20 making sure they hook up to my phone properly.

Plus I have epilepsy and my seizures increase when I have long calls with the phone against my face or with TWS earbuds in, which can apparently (and not too uncommonly) be triggered by the type of radiation they put off, even though it's at very low levels. But just 3 inches further from my face and no problems. I know that's not why a lot of people do it, and I still try not to in public, but there are various reasons that someone might.