You might be able to enforce a ‘voluntary code of conduct’ amongst experienced players or those who favour quality of game over just winning but if the weakness exists for a team to exploit to their advantage then people will do it. You can’t really stop them, after all if the game allows it to happen then it’s effectively within the rules. The only way to prevent it is to make it physically impossible to by restrictions in the game.
IMO there is no one single reason why it happens but map and game design are certainly at the core of it. People have highlighted situations like the axis spawn on railgun and the overpowering use of airstrikes as the worst offenders and I’d agree but perhaps it’s a more fundamental issue than that.
ET’s levels are based around trying to make an assymetrical situation - e.g. one team attacks and one defends - as fair as possible over the course of a game. The balance of power shifts as the teams progress (or don’t) through the level and in ideal situation, all other things being equal, skilled play would delicately tip the balance in either teams favour.
In reality what can occur is massive and decisive swings in favour of one team or another due to team imbalances, mistakes and exploitation or susceptibility to a flaw in the structure of the game. In truth there are so many factors to take into account when designing a game or level that it is impossible - ok - really, really difficult to make a level playing field.
Part of this sort of game’s appeal is that it provides such an open and varied environment and consequently that there is no single, fool proof way to win or complete a level. All you can really hope for is that glaring flaws have been spotted and removed as much as possible.
In short: It is the designers fault but you try doing it.
