First off. Define flat. Do you want flat over the entire sole of the plane, or flat within certain areas. What sort of value are you looking for here. There are two things that drive this. Market forces, and the law of diminishing returns.
Market forces is the customers who want high quality products at low quality prices. Festool, Mafell etc make very high quality products, but a lot of people (for many reasons) buy dewalt or makita because they are cheaper. Wall street does a lot of damage to the quality of tools, by driving maximum profit and shareholder dividends. This drives cost reduction and tolerance relaxation in manufacturing. (StumpyNubs does a great deal of promo work for small, family owned companies. These guys are more likely to put quality first, but you will pay a premium for it.)
The law of diminishing returns is related to the last point. A wider tolerance reduces scrap and waste, increases efficiency and fill rate and that satisfies consumer demand. The more we tighten tolerances, the more waste we generate (yes, there are possibilities to change and improve processes, but these also require investment and an appropriate return on investment, that more often than not is measured in time rather than expected revenue), the higher the cost of that manufacturing line, the higher the cost of each individual product, the less likely the consumer will pay that extra, and so on.
The question should really be, why do we buy poor quality planes and expect to not have to fettle them. I will lay money that our predecessors did not expect that.
I seem to remember in one of Pauls videos that he mentioned spending a weeks wages as an apprentice on one tool. I will also lay money that he expected to have to fettle that tool too.