well yeah but DIB has a flinch chance, hits twice, and is physical. have you seen melmetal's special stats?
my man's forgot the phys/special split!!
but yes in summary Steel Beam is a physical move in this meta just like all other Steel-type moves so youre comparing a 60bp STAB option that hits twice vs. a 140 bp STAB option that almost all pokemon of that type get, the maths should speak for themselves.
also, Steel Beam lacks recoil damage in this gen so it's a 140BP STAB option with no drawbacks, it's a great fit even on Corviknight to deal respectable damage to pratically anything it's looking at.
flinch rate is interesting specially with the popularity of para but probably not near enough to warrant a niche in this meta, if it does sadly Mel still really doesnt enjoy the other Steel types in the tier who provide more value overall; Corv has Roost, the advantages of a Flying type, and U-turn, Iron Treads has Volt Switch, way higher Speed (which grants it a good crit rate in gen 1), and advantages of being part Ground in being immune to T-wave and having STAB EQ, where Mel literally only sits down and hopes to beat you down with Steel STAB instead of providing any other sort of value, it's sadly pathetic movepool leaves it wanting for a lot in this meta
i don't get the reasoning behind steel and fire not resisting fairy, that's never been a thing before and all it does is make fairy types stronger
i'm fairly confident you didn't play this meta very much and either don't intend to, or don't intend on researching RBY mechanics first; Fairy is barely a meaningfully good typing in this meta for anything other than being immune to Dragon - Fairy moves are all physical in this meta and with most of them being special attackers you probably can tell they have STAB issues similar to most Ghost types in the past gens, let alone being one of the few types weak to Steel being very problematic with the aforementioned extremely safe Steel Beam
and if you're thinking this is deliberately chosen, this is literally how these mechanics work in Showdown backporting these moves to gen 1, the council didn't make any of this up, this is simply exactly how it works for some reason