Here's why:
* Uncountable: You can't count individual instances of "documentation". It's a collective term for a body of written or printed material.
* No plural form: You wouldn't say "documentations" to refer to multiple instances of documentation.
* Used with singular verbs: "The documentation is complete."
However:
There are instances where you might use "documentation" as a countable noun, referring to individual pieces of documentation. For example:
* "We need to update all the documentations for this project." (This is technically grammatically incorrect, but commonly used in informal contexts)
In general, it's safe to treat "documentation" as a mass noun.