Quote from bebepanales;34224:
but if you buy the permanent one, you only have to spend money once :P buying them weekly adds up.
Considering that it is a game feature that character grows every Saturday at a set amount of time, creating a niche item that tries to circumvent such game feature isn't very plausible.
Also, the idea of buying them weekly to give them more money is a better idea in terms of business. It also gives you more control when you want to change your mind. Can you imagine buying a expensive permanent one only to want to change your mind later? Then, they will have to create another item to reverse it.