I've looked through the PHB, and all over the SRD, but I can't find anything. What I'm wondering is, do you get your Strength modifier as a bonus when making a touch attack in melee?
Does it sound like a melee attack roll? Hmmmm, guess it does! So STR applies.
It can be argued that DEX would serve us better in this case, but Str would likely prevent your being fended off to some degree.
Dex on ranged touch attacks, of course.
This being said, we're you actually really looking for a rational explanation relative to a "fantasy" game that does not even begin to pretend it wants to be a simulation?
I consider that the purview of Weapon Finesse. If low strength, high dex characters (or pitiful Str, slightly above average dex mages) want to hit in melee, they should take the feat. Why give something away for free if people will pay for it?