Here's why:
* It's a collective noun: It represents a group of items (chairs, tables, beds, etc.) collectively.
* It's not something you can count individually: You can count the number of chairs, tables, or beds, but not the number of furniture.
* It's treated as a singular entity: You use singular verbs and pronouns with it: "The furniture is beautiful."
While you can count the number of pieces of furniture, the word "furniture" itself isn't meant to be counted. It's a general term describing the category of objects that serve a particular purpose (e.g., seating, storage, support) within a home or office.
In short, furniture is uncountable because it represents a broad, collective concept, not individual items.