Nope, its because Carrick is still a very good player.
As for Fletcher, the reason he's not playing week in week out is because he's not good enough.
Gibson was the same.
Park also the same.
Anderson also the same.
None of the players above are/were defensive midfielders (with the exception of Carrick who can fill the roll with merit), so comparing them with Parker is apples and oranges.
As for the argument as to whether Parker would walk into the team, he would. As he is arguably one of the best defensive midfielders in the Prem and we have coveted a great DM for years. Hargreaves fitted the bill but was a gamble on medical grounds, we tossed the coin and lost.
As to the argument as to who is the better player, there isn't one, they are not the same, so the whole debate is moot.
However, IF i had to choose one player that "I personally believe" would benefit Manchester United the most between Parker, Park or Fletcher.It would be Parker hounds down as he is the "type" of player we need the most and not because he is the best.