Why dont games have Weapon modifiers

I like the idea of users/players be allowed to change weapon whichever way they want - being able to change Rof, Damage, Range etc would allow us to create many new types of gametypes out of a single game.

What I want to know is what do you think is the reason none of the games (or to be specific Halo) have these options.
One of the reasons I think is that it would be hard to implement graphics wise - for eg DMR firing at 200 round per second would be graphically very different than firing 2 rounds per second. These kind of changes would need to be implemented at coding level and cant just be supported at scripting level.

Please let know if you think that there might be some reasons like marketing of game etc which would be affect by adding weapon modifier support.

I basically don’t like the idea.

I just don’t see it working, in any way. A DMR firing 200 rounds per second, it’s just not a DMR anymore. Would be actually a instant kill weapon, even with bloom. Is impossible to use it in MM, and don’t why it should be on Custom Games.