Here's why:
* It refers to a mass of undifferentiated material: We don't usually talk about individual pieces of garbage, but rather a collection of it.
* It's not typically counted: We don't say "one garbage" or "two garbages."
* It's used with singular verbs: "The garbage is overflowing."
However, there are some exceptions:
* In specific contexts, you might use it in a countable way: For example, you might say "They collected five bags of garbage."
* "Garbage" can be used figuratively to refer to something worthless: In this case, it might be treated as countable: "He spewed a lot of garbage about his political opponents."
Overall, while "garbage" can be treated as countable in specific contexts, it is generally considered an uncountable noun.