It's because a large percentage of the United States, and other developed countries, have debit and credit cards. The more people using a cryptocurrency the better, so developers want to sell to a larger portion of people. A majority of people will not buy a bike or car that generates cryptocurrency. Its sad but a real fact. I, along with the majority of people, spend USD through a debit card. If you are making a currency to replace USD, then you must replace debit and credit cards. But your ideas are good and interesting concepts.