Shields are reason ...
Since their AC bonus is counted if you have equipped one, rather than holding it in your hand.
So if you would have Weapon 1 (something + shield) and Weapon 2 (2h weapon) ...
You would have 2h weapon AND +2 AC ...
Not sure how hard it would be to fix this ...
But IF it would be fi... lets call it changed.

So IF it would be changed ... so your AC bonus from shield would only count if you are actively wielding it in your offhand ...
I would like to remind my old wish for Martial classes to have additional weapon slot(s) ...
I mean, even Fighter in player manual (im not sure if its even possible to get more official info than this picture)
![[Linked Image from dndbeyond.com]](https://www.dndbeyond.com/avatars/thumbnails/6/359/420/618/636272697874197438.png)
CLEARLY have multiple weapons prepared for immediate use!
And it would feel really good if our Fighter, or Barbarian, could be prepared for several types of enemies. :3
Also ... this third (or even fourth?) slot can still be usefull for NON-martial classes ... my only suggested restriction here would be to not allow then to put weapons there ... only torches, candles, maybe shovel (its quite anoying if you accidentaly sell it from your inventory) ... and simmilar stuff.
//Edit:
On second thought tho, maybe let everyone equip mutiple sets of prepared weapons ... after all, casters would still need proficiency to make shield usefull ... and beyond that, rarely any weapon is more effective than spells.

So ... there would be no harm, i gues.