Because you can't take a bike frame to "most autobody repair shops [that] have welders and have him reweld the bike". What you are likely to get back is a bike with a hole where a weld used to be. The steel on bicycles, including cheap frames, is incredibly thin and easy to burn through. It is a myth that a steel bike can be repaired by anyone with a welder.

A broken weld is also a stress riser which will lead to further damage down the road. The very best course is the one that dabrucru is taking, i.e. contacting the dealer.
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