Chunk of a Boeing 737 fell off United flight, this one from San Francisco to southern Oregon

[Aussie105]

14. Planes are complex machines.

And like any machine, as they age and knock up the miles, problems develop that the designers never thought of or are checked during routine maintenance.

Probably more profitable to keep them flying and deal with problems as they arise, rather than have them on the ground not earning money while routine maintenance is done in more detail.

Be honest - who drives their car with just basic maintenance and have things go wrong unexpectedly?
Most of us, I daresay.