I agree, but the investment mechanics/rules for buildings/wonders had to be kept identical in order for the AI to be able to understand how to use it. Plus, there was the justification that "investing" infers that you are putting money into a project early in order to speed up production, and thus it doesn't make sense to be "investing" in a project that is already near completion. But the "IRL" reason aside, the real reason for the new mechanic was to prevent "production overflow via investment" strategies from being a common part of high-level play, since the AI is completely incapable of understanding and performing them.