It's all about dollars! They went from forged to cast cranks to save money, cast cranks are lighter than forged so to have acceptable balance, weight would need to be added. Mallory metal is very expensive plus it adds extra operations that also add cost.
Since a cast crank motor with external balance is more than adequate for the purpose that is how we get to where we are. Remember, Detroit does not design cars for the 1/2% of folks that want to do engine swaps or do other hot rodding activities.
Virtually everything we touch is a compromise between cost, function and reliability. Any engineer worth his salary is looking to remove cost without impacting function or reliability. Since an extrernally balanced motor does not impact fuction or reliability for a daily driven vehicle but can make a very significant reduction in cost it was absolutely the right decision to be made.
Having been an engineer for the past 32 years I am faced every day with how best to make the compromises for the target market. Many times those choices are not what I would make if I was designing a product for myself but my job and salary depends on providing a product that the largest number of people want to purchase.