One of the key points to answer your question is thermal stress.
When you apply heat to one pin of a device, there is a sudden and huge temperature difference between that point and the rest of the device. That difference is stress, and the result can be a material breakout.
In an oven, on the other hand, all the board is put under a controlled, gradual thermal rise. ALL the points of the device are at almost the same temperature, so there are no thermal stresses (or they are much smaller than) they were when you applied the soldering tool to ONE pin and the rest of the device is at room temperature.
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