Ttyjfyk

The Iron Man suit can be manufactured in mass easily. The Avengers initiative is to defend Earth from imminent global threats that are beyond the warfighting capability of conventional military forces.

It will definitely help to have thousands of "men in the advanced suit" to fight alien-like threats. Why don't we see such a military unit in the MCU?

The short answer is: accountability. After Obadiah Stane's betrayal during Iron Man 1, Tony doesn't trust anyone else with the Iron Man technology (except his best friend, James Rhodes), because he does not want to risk it being used for the sort of atrocities the Ten Rings used his weapons for in Iron Man 1. This is also the reason he shuts down Stark Industries' weapons division.

This idea of accountability, or of others getting their hands on the Iron Man tech, comes up time and time again throughout the Iron Man trilogy:

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  At the climax of Iron Man 1, Obadiah Stane builds and pilots the Iron Monger suit based on Tony's original prototype. This drives home to Tony just how dangerous the Iron Man tech could be in the wrong hands:

  
  
  

  
  

  OBADIAH: Ironic, isn't it, Tony? You tried to rid the world of weapons; you gave it its best one yet. And now, I'm gonna kill you with it.

  
  

  
  



• The Senate Committee scene in Iron Man 2 is the US government asking Tony to turn over the Iron Man suit, presumably so they can mass-produce them. Tony point-blank refuses, because he knows that's what they're planning to do and doesn't want to be held accountable for what the government might do with them. (And just to justify Tony's paranoia even more, Captain America: The Winter Soldier reveals that the head of the committee, Senator Stern, worked for HYDRA.)



• Then, later in Iron Man 2, the US military do get their hands on the Iron Man tech when James Rhodes brings them the Mk.II suit. Justin Hammer gets the contract to develop the mass-produced suits, and gets Ivan Vanko/Whiplash to assist him. This, of course, ends with Vanko hijacking the suits (including the one Rhodes is inside) and sending them on a violent rampage as part of his campaign to destroy Tony's legacy. This would only reinforce Tony's refusal to militarise the Iron Man program - what if those suits got hacked?


• Interestingly, in Iron Man 3, there actually sort of is an Iron Man army - Tony has built dozens of suits that can all be remotely operated by JARVIS. However, if I recall correctly, he didn't build them specifically to serve as an army in case of a serious threat, but as a coping mechanism after the events of The Avengers left him with PTSD. At the end of the film, having come to terms with his trauma and vowed to prioritise his relationship with Pepper Potts over his Iron Man career, Tony destroys the surviving suits.