"Why not just have 4 smaller engines instead of 2 big ones?"
That's a fair question. It's because efficiency scales with engine size. The purpose of the 737 MAX was to be more efficient.
Also, cost scales roughly with number of engines. Unless the half-size engine is made with half the number of parts, manufacturing isn't radically cheaper. Cost of maintenance doesn't change much with size, either. And with more engines, you have more engine-hours per flight-hour, and thus a higher rate of engine problems per flight-hour.
If Boeing had built a "747 MAX" instead, maybe we wouldn't have these problems. Airlines wanted a cheap twin-jet, though, so that's what Boeing made. They just cut costs in some really stupid ways.