Phone calls are synchronous. You have each others attention, questions can be answered quickly. Also the big one is that hard to answer questions will die in an async setting. What's your current salary, why did you leave your last job so soon, anything that creates a desire to procrastinate with an answer.
Questions like these may be appropriate¹ to ask in a scheduled interview. It's completely not respectful to pepper a person's week with multiple unscheduled "quick chats" or callbacks. If they want to schedule something that's what calendly is for.
¹The salary question is illegal in some places and kinda shady in the others.
