For the same reason every clothing department store has a changing room. People like to try things on, see what pairs with what. The current try on feature is good but it limits combinations.
Expanding this feature to Operative menu would allow players to equip premium cosmetics while in the Operative menu, then as soon as the player exits the menu the cosmetics can be unequipped. That way they can mix match weapon cosmetics with character cosmetics and it could increase sales.
Btw, the addition of cosmetic packs and more cosmetic options in the Commodore Vestures is a good step in the right direction, thank you to whoever implemented that change. It’s not an open catalog but it’s better than it was before.