But the question is...
...why ISPs don't allow users to opt for dynamic addresses (at less cost to the ISPs)...? (Emphasis added)
I don't doubt that they could charge enough for the cheaper service to keep it at least as profitable as the more expensive service.
If, as suggested, dynamic IP addresses are ISPs' defaults, then the only "option" is for static – at a premium. The cost of the former is probably dictated by market conditions in every arena where service is offered.
