Regulators probe engine blow-outs as older Boeing 777s suspended

[DeminPennswoods]

8. The engine is hot and spins at tremendous speed

Even the best engineered and manufactured parts can fail under that kind of stress. But there are also occassional problems with microscopic impurities in the castings and forgings from which blades are made. It doesn't take much for that to evolve into a failure that could be isolated and specific.

Money is the biggest driver. It's expensive to pull an engine, inspect and replace blades or vanes or disks. The blades come in matched, balanced sets. You can't just replace 1 blade, you have to replace both lest you put the fan or compressor stage out of balance.