Why might a weld be weaker than a bolt?
Under perfect conditions this is not true. However most of us are unable to weld something under perfect conditions.  There is always the risk of contaminants, imperfect welds, incorrect temperatures, etc.

This holds true even in many industrial factories. For this reason, some very high end cars have their frames glued together instead of welded.

You might have heard this under the assumption that most people don't do a great job at welding and as a result, a bolt is less likely to fail and easier to recommend.  Of course this also assumes that the company who made the bolt has good quality control too and they always hold the forces they are supposed to (unlikely in the real world with the typical low quality stuff we buy).