Here's why:
* Collective nouns refer to a group of people, animals, or things considered as a unit.
* "Colleagues" refers to a group of people who work together.
* The phrase "with colleagues" emphasizes the group aspect, even though it's used with a preposition.
Examples:
* "The team of colleagues worked diligently on the project." (Here, "team" is the collective noun, and "colleagues" defines the group.)
* "I went to lunch with my colleagues." (Here, "colleagues" is the collective noun.)
Even though you can also say "I went to lunch with my coworker," "with colleagues" emphasizes the group nature of the gathering.