system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek] why didn’t Starfleet keep a group of MACOs around?

The MACOs were military personnel and the group was disbanded after the founding of the federation, with a few people getting Starfleet commissions. With the preponderance of military operations that Starfleet engaged in throughout the 22nd, 23rd, and 24th centuries, wouldn’t a branch of highly trained and effective Marines be useful to hold in reserve? 

Lets face it, the Earth-Romulan war, the first and second Federation-Klingon war, the Logic Extremist campaign, the Battle of Axanar, the Federation-Sheliac conflict, the battle of the Mutara Nebula, the Federation-Cardassian war, the Federation-Tholian conflict, the Federation-Tzenkethi war, the Maquis insurrection, the Dominion war, the Borg conflict, and several more could have been aided by dedicated military personnel.

Starfleet is definitely a group of scientists, explorers, and diplomats, but they engage in hot conflicts enough that a military organization in the Federation might be useful.
gpt
Because Federation has an allergy to anything that is explicitly and solely for military. You see it in their starship designs and commanding offices. All ships come with science labs/recreational areas/daycares and Captains are often explorers, scientist, or diplomats.

Until Sisko realized this kind of thinking is stupid and created the Defiant.