Building on this comment as a mech eng who works in metal construction of heavy industry, it's also to do with the economies of scale and profit margins. As yo

Building on this comment as a mech eng who works in metal construction of heavy industry, it's also to do with the economies of scale and profit margins. As you reduce the purchase price, you need to increase volume to ensure profits. Similarly, you want to absolutely minimise construction cost to maximise said profit. As you increase volume and minimise construction cost, you rapidly reduce the scope of possible manufacturing techniques. You narrow down to tried and true methods of fabrication and assembly. Now the parts must be readily handled by robots, reproducably accurate between the same parts, parts must be able to swap between mating parts without bespoke tailoring (ie: the opposite to Apple's vision system to pick the right set of parts to assemble together), it must reliably assemble and stay assembled, it must be easily and quickly maintainable by cheaper labour for aftermarket cost minimisation, it must perform well in the battery of safety tests yet not become structurally complex to pass them, it must be light to minimse raw material costs, etc. etc. Deviating from this essentially adds cost, and even a small additional cost adds significant burden due to volumes and small margins. With all of those constraints in place, you then consider a middle-of-the-park design that isn't polarising in its appearance or function to maximise market share. Thus, you converge towards a relatively inoffensive sedan shape that Just Works for all parties and still satisfies that litany of constraints on the design, manufacture and maintenance.