Stronger doesn't mean stronger, it has to be both stronger, and in a situation where the limiting issue is the strength of the actual joining method.

Second, lugs weight more, so the weight spent on lugs, not to mention the time, needs to be the best use for those budget items. What would happen if the weight of the lugs were added to make the tubes large in diameter, and those tubes were welded.

You can't just generalize on BOs for racks. One is placing X number of BOs in particular places, on particular tubes, etc... Al that maters is that they are not compromising the strength of the bike where they are added. For instance the fork ones are mostly on the neutral axis, in beefed up tube ends, or drops. The rear ones are mostly not in the tubing. The thing that seems to break the most, are bolts, so designing the whole thing around larger bolts is probably more bang for your buck. However, for average uses the usual locations are pretty good.