Because a lot of people don't understand how the products work or what they need. Trying to understand a complex product, with multiple options that come with advantages and disadvantages, and having adding on top that it's a very consequential decision is much easier if any misunderstanding can be corrected and explained real-time. The alternative is to do it async with some lead time for every back and forth.

Your pizza order needs no clarifications and if you get it wrong, it's just a pizza. If you misunderstand your mortgage you're looking at far more costly consequences.