GS is not random at all. There are many threads that give advice on how to manager and maintain GS. GS can be managed, and, although it can be difficult and non-sensical sometimes, as a good manager, you should be able to keep your best players at Strong/Proficient game shape for most of the season. (with the exception of the first week).
The argument to changing the GS reset to become random at the start of the year, is that it would introduce elements of uncontrollable randomness into the game. Which would suck.
Like some of the examples given, if your best player comes back from off-season training with bad game shape, and it takes you 4 weeks to get him back to respectable/strong GS, and you lose all 8 games in those 4 weeks because of this players poor game shape, then that is a pretty darn good argument as to why its absurd.
Look at it from a different perspective. In my league, several teams do not know how to manage game shape, and as such, their players under-perform. So their players under-perform because their managers don't know what they are doing. Poor management = poor performance. Good management = good performance. Just like all other aspects of this game.
If you introduced a concept into the game where a player masively underperforms due to some totally random uncontrollable factor, then you would probably see a lot of users not play this game.