Why is closing the center by White generally considered a concession? I mean, pushing e4-e5 when there are Black Pawns on d5 (and e6), or pushing d4-d5 when there are Pawns on e5 (and d6), is often considered "the second best way" to play, and something that White has to do only if necessary. I.e. vs King's Indian is considered generally a stronger opening strategy to keep the d4-e5 tension (by delaying d4-d5) until necessary. Why?
Based on my understanding it's not closing the position that is a concession, it's releasing the tension for no reason that is the true concession. Finding meaningful moves in a position while maintaining tension, whether that is not moving pawns in contact that can take each other or pieces in contact, is approaching master level play (I say this being a master myself). The only time you want to release tension is when you get some tangible benefit. The results of releasing tension too soon is usually just slightly worsening your position, but can result in an outright blunder occasionally.
Concerning the Kings Indian you mentioned in the Mar del Plata variation where black plays 7...Nc6, closing the center with 8. d5 is the most "correct" move as you gain a tempo and gain space by releasing the tension. This makes releasing the tension completely worth it. In the Petrosian variation of the Kings Indian by comparison white only gains space by releasing the tension, which while not that bad often leads to equal positions with best play, losing the advantage of the first move. In systems of the Kings Indian where black plays Nbd7, often white is served best by maintaining pawn tension until black plays Re8 or Qe7 to prepare to play exd4 and try to gain piece activity along the e-file, then he can release the tension with d5, gaining space and pointing out black wasted a move to misplace his rook/queen to a file that will never be open.
You could say the philosophical reason to maintain the tension stems from forcing your opponent to develop his pieces and plan his moves in such a way that he has to react to a much larger series of positions and plans from yourself. This helps limit his pieces and keeps them from being placed optimally and efficiently. Whoever released tension first generally is simplifying the game for his opponent and making it easier for him to maximally place his pieces. Hence releasing tension must be justified with some sort of benefit, or because the tension just can't be maintained since the opponent has too much pressure on your position (understanding when this point is reached can be very difficult).
So closing the center itself isn't bad generally, it just depends if it's the right time to release tension or not.
White has the advantage of the initial move. As such, he will want to preserve as many options as possible (all other things being equal).
"Closing" a position reduces those options (by definition). That's why Black will sometimes play e6 and d5 (French defense) in response to d4, to try to induce White to close the position with e5. Some players feel that the advanced pawn in e5 is sufficient compensation, others do not.