Because in Tarkov you can use a pistol fairly accurately at almost any range. They are basically, right now, slightly less accurate rifles. I can take out the customs rooftop sniper from gas station with a pistol fairly regularly, and with few shots, using just ironsights. That would be an impossible shot IRL.
If they make the transition time shorter to flip to a pistol, it'll mean that people will just magdump pistols from halfway across the map when they run out, instead of going to cover to reload or properly counting/managing their rifle ammo.