More precisely, airplanes are designed to being 50% stronger than the max load it would ever see. Rockets are designed to something like 10% stronger.
But my point here actually was the difference between "at least x times stronger than it needs to be", and "exactly x times stronger than it needs to be". Usually things are manufactured to the first spec, rockets are much closer to the latter spec because keeping weight down is so important.
I suppose airplanes must also be closer to the latter spec than most things, but they should still be more forgiving than rockets.
What you're describing is how farm tractors are repaired, not how aerospace mfg. is conducted.
You can't "test out" bad metallurgy, welding or mfg. in rockets.
You are far from the only one who is skeptical that this is a valuable process, but there isn't any room to debate that it is what they are doing.
Edit: Here is a recent timelapse of some of that welding, as an example: https://www.youtube.com/watch?v=J7bIvLhI1hY
There's a reason SpaceX is eating "big aerospace's" lunch and tests like this are part of it.
A prototype that was manufactured in a tent and in a field, with welds being done in that field only a few days ago. I doubt much QA or weld level testing was done.
A prototype on which they knew welds were less than ideal...
And specifically a prototype of a rocket being built with new materials and new manufacturing methods. Rockets have the nearly unique distinction of having to be strong enough and no stronger because being stronger wastes mass which lowers the very important payload to orbit number. Bursting a few pressure vessels as they figure out where that point on the strength curve is seems pretty reasonable to me.
We don't (yet) know at what pressure this burst at, if it burst near the total pressure needed to achieve their safety margin I imagine they will be ecstatic. I wouldn't interpret this test failure as a process, or QA failure without more information.