For the same reason Boeing developed the 737 MAX instead of a completely new model. The fixed costs are too high, the risks are too high, and the profit margins are too low. If it takes 10-15 years to develop a new plane, several years to ramp up production, and several years beyond that to break even, it does not make sense as a business. If you think it purely as an investment, there are always better uses for your money.
Wasn't a significant part of Boeing's decision to "recycle" the 737 time-to-market? Sure, they saved a bunch of money not designing a new plane, but they were also under heavy threat from the A320neo and needed to get something out there relatively quickly.
