So?Reason being is
Greatsword = 1 attack at 2d6+Stat
Longsword + Shortsword = 2 attacks at 1d8+ Stat/ 1d6
TWF deals greater DPR than the Greatsword guy. 2 attacks instead of one, and combined damage of 1d8+1d6+stat.
'Light weapon required in the offhand only' might work as default, but only if you imposed disadvantage on the off hand weapon attack, unless the primary weapon is also light..
A d8 longsword (the best 1h weapon damage you can get) averages 1 pt of damage more per round (2 max) over a shortsword/shortsword combo, assuming you hit both times. Caeteris paribus, A dual-wielder would get an extra point of damage in exchange for the opportunity to use his bonus action and potentially miss one of his attacks and due less dpr than if he'd had hit just once with a 2h weapon.