Because (naively) I'd assume it's some kind of software running on computers, and one can generally improve performance of that by buying more hardware or using

Because (naively) I'd assume it's some kind of software running on computers, and one can generally improve performance of that by buying more hardware or using better technology (or shorter lived components, or better cooling, or whatever). If humans created an AI to solve some kind of open-ended problem, where "more AI" made the solution better, there would be every incentive to spend the money on more or better hardware for it. It's not clear that a shark would gain much by being 100x smarter, particularly if the metabolic cost were high; for a lot of human problems, spending 100x more on hardware/power/etc. for a 10% better solution would be quite desirable.