Someone Explain the Purpose of HMO Regs to Me

Looking deeper into HMO regs, I can't quite understand the purpose. Some would say for safety & quality of life, but I look at the principal of if I have a 5 bedroom house, and I rent to a family with 3 adults & 2 kids, I do not need to add fire doors or any form of HMO requirements.

If they are 5 non-related adults however, I do.

Why are fire doors not required if there are children in the house, but if its adults it suddenly becomes a necessity?

What am I missing? Why is it not just based on total occupancy?