*) If you request a concede button for multiplayer games that immediately stops the game and declares the other players as winners then you actually request the current implementation.
Since there is - in general - no way to know the outcome of the game if you stop it inbetween you cannot rank the other players - thus the best solution is to tie them to each other and give them a win over you. Since you ended the game prematurely, others should be warned that you might do it again. Thus you should receive a Karma penalty.
*) If you request a concede button that does not stop the game and lets the other players continue (and lets the quitting player make null-moves) then you require quite some programming work by BGA and by its volunteers.
First of all, at the moment (almost?) all multiplayer games on BGA are treated equal - no conceding possible. At your request, some of these games would continue and BGA has to change the general site rules how to rank these games (ELO, win rate,...). Other games have no neutral "null moves" that an absent player could take according to the game rules (e.g. Agricola). These games, which are many, have to be treated separately because they cannot continue after one player leaves.
And even with Catan you have to change the programming when a player concedes.
-) You want him to roll the dice and do nothing. But you do not want the other players to have to wait, thus you have to newly program the clock.
-) If a 7 is rolled the active player has to move the robber. A null move is not allowed by the rules. If the active player is absent and any decision is made by BGA the game rules are not followed anymore. If the robber is put on the desert this is illegal since the game decided it, not the player itself. And what do you do if the robber is currently standing on the desert and all hexes have settlements beside them? Have a democratic vote among the remaining players where to place the robber?
-) In case of a 7 the absent player may have to discard half of his hand cards. The choice has an effect on the remaining players because they may rob later. So who makes the decision what cards the absent player is discarding? There is no fair algorithm (random, oldest cards, newest cards, most valuable cards,...) for that procedure.
All in all, Catan allows to replace an absent player by some minor tweaks of the rules. But since BGA is not allowed to implement house rules without consent of the game publisher BGA is not allowed to do so. And in general, game authors do not write their rules including the case that a player is leaving the game prematurely. Over the board at home you might come up with house rules. This is not possible at BGA.
I hope I gave you some insights why currently there is no conceding at multiplayer games. There might be better solutions but you did not suggest a better algorithm that handles the problems I have stated above.
gg
