cheaper equipment: more plastic (and not the good kind), less stainless steel, less precision, poorer seals, lower grade metals... etc.
sora STI levers just don't seem to last as long as 105/ultegra.
If there is any stainless steel inside of the STI's, it's confined to the fasteners, pins, and springs. All of the various plates and cams are steel (per the Dura Ace and Ultegra STIs I've disassembled). The pair of Sora STIs I used for several thousand miles felt as good as new when I got rid of the bike (no different than my experience with Ultegra STIs). None of the STIs I've looked at or disassembled have had much in the way of sealing. I also wonder how you determined what grade of steel/aluminum was used to produce the various STIs. Sure, the higher end stuff is lighter but that doesn't mean much in the way of quality or durability.