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I guess that's because the manufacturers would prefer to play it safe, rather than get bad press of failing frame under heavy riders riding outside of the 90th percentile conditions. Case in point is all road genesis, for which I believe they're using CdF tubes, when in all honesty they could probably get away with much lighter gauges 99% of the time. A custom builder know exactly for who and what style of riding they're building.
tl;dr what he said.
The other point is that there is a big difference between mass produced 853 and hand-build 853.
Even thought I had a mass produced decent steel frameset, it still feel overbuild and dull* compare to a lovely custom steel frameset, especially the fork.