The Cloak of Elvenkind requires an action to pull the hood up or down, which gives you the benefits of the item. In the PH page 190 in the side bar it explains examples of things you can do in tandem with your movement and action, one of which is pull the hood of your cloak up and over your head. So than why does the Cloak of Elvenkind require an action?
2 Answers
In the PHB on page 193, under Actions in combat -> Use an object:
You normally interact with an object while doing something else, such as when you draw a sword as part of an attack. When an object requires your action for its use, you take the Use an Object action. This action is also useful when you want to interact with more than one object on your turn.
As to why the Cloak of Elven Kind requires you to do that? Magic.
If you feel the need to rationalize this behavior for the purposes of roleplay, you can imagine the magical effects of the cloak to take a few seconds to set in, much like natural camouflaging animals take a moment to change their appearance.
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\$\begingroup\$ In general, the "Use an Object" action is for nonmagical item usage; based on designer statements (and, I believe, the version of the rules included in the Essentials Kit), it seems like magic items that require using an action are typically associated with their own separate kind of action. \$\endgroup\$– V2BlastCommented Nov 15, 2022 at 17:31
The reason is game balance. The designers must have felt that activating the cloak on the same combat turn as attacking or casting a spell was too powerful.
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2\$\begingroup\$ This answer could be improved by providing some support. Either statements from the authors that support it, or a more concrete analysis of the in-game effects showing that not requiring an action would be unbalanced. \$\endgroup\$ Commented Nov 15, 2022 at 16:12