A straight line is the shortest distance between two points. A straight line is also the best way to transmit most loads.

Some bikes use elevated chainstays. Some are very elevated. The frame thus does not resist chain tension with a straight line. Instead, the load is highly "cantilevered". That is, the frame reaches "out and around" to support the load, instead of using a straight line between the cranks and rear wheel. A highly-cantilevered frame is the moral equivalent of a P.M.P. crank:

There are some good reasons to build a cantilevered frame: supporting the wheel above the chainline means the frame can clear a wide tire without crowding the chainrings and thus avoids compromises like a wide bottom bracket and wide "Q". A cantilevered frame can reach to a suspension point that may give a better ride than a similar design with the pivot behind the bottom bracket.

That said, the drive chain is often under tensions several times the riders weight. Reaching "out and around" means the chain tension has a lot of leverage to bend the frame. As the chain pulls the back axle towards the cranks, the frame flexes, with every pedal stroke. Unless the frame is very sturdily built, repeated flexing damages the frame. Historically, highly-cantilevered frames tend to break.

They also tend to be flexy, but that's usually a minor problem compared to falling apart.