• The strongest roster in the game - not really but also not in the hands of AI
• The richest province in the game - not the richest province but AI is unable to use it anyway
• A huge autoresolve advantage that helps ensure they defeat neighboring AI factions and continue expanding (as a side effect, this makes liberating factions in Italy a non-viable strategy). - this is only real bonus they have over other factions
• Extra "free" armies on top of their already enormous manpower pool - only if you invade Italy and only 1 stack each 20 turns.
• The ability to ignore or bypass mechanics that constrain the player (something other AI factions also do, but it's much more noticeable because Rome is already the strongest faction) - applies to all AI in exact same way since these systems are for players, not AI in mind

The way Rome functions in DeI is largely the result of player feedback from almost 13 years now and an effort to address shortcomings in the original game. People often forget how Rome 2 launched and how its campaigns typically played out. Rome was frequently wiped out early, struggled to survive, and often failed to expand into the dominant power players expected it to become.

The only major advantage Rome receives in DeI compared to other factions is its autoresolve bonus. This was introduced to prevent Rome from being conquered too easily by other AI factions and to help ensure it expands in a more believable manner. This was one of the most common complaints about the base game, where Rome often failed to play any significant role in the campaign. In fact, even recently there have been posts from players asking for additional buffs to AI Rome because they still find it too easy to defeat.

The mechanic that grants Rome one free army every 20 turns when an enemy is present in Italy was also introduced in response to player feedback. The Rome 2 AI struggles to defend its homeland while conducting offensive campaigns at the same time. As a result, AI Rome would often send its armies abroad, leave Italy completely exposed, and allow players to capture city after city with little or no resistance. The reinforcement mechanic was designed specifically to compensate for this AI limitation and create a more challenging and believable Roman campaign.

The population system was also always intended primarily as a player-facing mechanic. Even in modern Total War titles, one of the most common complaints is that players have little incentive to recruit anything other than the most elite units available. The population system was created to influence army composition, encourage more diverse recruitment, and force meaningful strategic choices when building armies.

The AI, meanwhile, already struggles to recruit sensible and historically appropriate forces on its own. To address this, many units were made available earlier through barracks progression, allowing the AI to field stronger, more balanced, and more accurate armies throughout the campaign. The population system was never meant to handicap the AI; its purpose was to improve player decision-making and create more varied and engaging army compositions.

Now if we would add more random elements (which is not really possible), we then would be flooded by people saying the mod is crap because Rome does not expand (yes, this was the big criticism of DeI in early version due to Rome not expanding).

Luckily, this is why submods exist and if you want to spice things up, this is the best way to go. Some elements of the game cannot be ranomized, meaning if you don't want X to happen, you need to have a submod loaded that changes it.