Avoiding collection of entities

[Eslam Elbyaly]

Hi,
I am studying systems analysis and design from a book written by Jan L. Harrington "Relational Database Design and Implementation, Fourth Edition 4th Edition", and I can not understand the following point,

AVOIDING COLLECTIONS OF ENTITIES:-
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When you first begin to work with entities, the nature of an entity can be somewhat confusing. Consider, for example, the merchandise inventory handled by Antique Opticals. Is “inventory” an entity? No. Inventory is a collection of the merchandise items handled by the store. The entity is actually the merchandise item. Viewing all of the instances of the merchandise item entity as a whole provides the inventory.
To make this a bit clearer, consider the attributes you would need if you decided to include an inventory entity: merchandise item number, item title, number in stock, retail price, and so on. But because you are trying to describe an entire inventory with a single entity, you need multiple values for each of those attributes. As you read earlier, however, attributes should not be multivalued. This tells you that inventory cannot stand as an entity. It must be represented as a collection of instances of a merchandise item entity.
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Can you explain why "Inventory" can not be an entity?