Because it costs money and Boeing would rather have larger executive bonuses than pay for a new design. The fact that a new design would be better for the futu

Because it costs money and Boeing would rather have larger executive bonuses than pay for a new design. The fact that a new design would be better for the future health of the company is irrelevant: the executives won't be there in 20-40 years, so why should they care about that? All that's important is their compensation in the next few years until they land a gig somewhere else.