Why shouldn't (Shadowrun) be designed to model a fat nerd knife-fighting a SAS commando..I feel previous editions modeled it adequately.
Because Shadowrun is a game where you play Shadowrunners doing Shadowruns. In-universe, there are firefighters putting out fires. Athletes playing sports. DocWagon medics treating sick kiddos in urgent care clinics. Wageslaves working soul-crushing corp jobs. The rules aren't designed to cover any of these types of characters or activities, even though you might temporarily get involved in them for a single mission. Or even as a heavily modified, non-shadowrunner type of campaign.
The combat ability of a fat nerd is simply not relevant in a game about shadowrunners performing shadowruns. Allowing them the same DV when using weapons is well within the suspension of disbelief, imo. Especially since Edge is an even bigger X factor anyway than "professional vs non-professional" weapon familiarity.
And what happens when the fat nerd in question is the team's decker?
The decker never is the fat nerd. Even if the decker is fat, nerdy, and has identical physical attributes and skills, the "pedestrian" fat nerd is still not a professional shadowrunner. Even if they have identical stats, any shadowruner categorically a different thing thing than any NPC, to say nothing of a noncombatant NPC. Even while lacking the stats and technical skill of a Sammie, Deckers have a familiarity with how to "do" Shadowrun related activities that are not necessarily also true for NPC fat nerdy types.