There is some truth to this. D&D is and always has been a game about killing stuff to aquire loot and personal power through 'leveling up.'
The game mechanics support this and only this. There is no reward for 'heroism' or charity or kindness or mercy or self-sacrifice other than those the GM artificially imposes (and, indeed, earlier editions were even more strongly weighted in this direction, with gold pieces directly equally experience points, making it in the absolute best interests of a PC to hoard more than his fair share of found loot, and the XP penalties of death, and limited / selfish nature of 'save myself' options like dimension door or word of recall, discouraging heroics or brave stands to hold back threats to allies quite strongly).
In games like Mutants & Masterminds, or GURPS, aquisition of powers comes much slower after character generation, and starting characters may be powerful and 'heroic' right from the start, encouraging riskier behavior (since they aren't perpetually one lucky crit away from death, as a starting D&D character is). There is less motivation to hoard resources, or focus on 'getting that next level,' and, depending on the system, much less focus on 'gearing up' (particularly in Mutants & Masterminds, where 'loot' is practically meaningless).
You *can* make all sorts of campaign choices (rewarding heroic characters in-story, denying rewards to more selfish characters), but it's very much swimming upstream of the base D&D assumptions.
A focus on classes or concepts that promote teamwork or reward selfless behavior (such as the Marshall class, for teamwork) might be useful, and trying to 'defocus' on classes or concepts that 'don't play well with others,' like stealth-focused characters (who, by dint of the stealth rules, are required to stay the hell away from their allies and go off alone all the time).
Many games have these sorts of 'loners,' separated by role or sub-system from their party, such as cyberdeckers in Shadowrun (wee, let's all stand around holding our cheese while Decker Sam has a solo adventure in the Net!) or characters with unique movement types (wee, let's go out for pizza while the aquatic elf / aaraokocra / etc. goes and explores a place that the rest of us can't reach!).
MMOs sometimes attempt to deal with this sort of 'selfish powers' mentality by arranging for class abilities or buffs to extend to all groupmates as well, making the group synergistically more effective in total, than as a bunch of individuals, or by making 'escape' or 'evac' abilities affect the entire group, so that a single character isn't encouraged to desert the rest of his group via word of recall or expeditious retreat or a wicked fast monk run speed or hide in plain sight or whatever. Tweaking D&D to reward teamwork and not abandoning fallen allies, etc. would require some serious changes to mechanical assumptions that have been in place for decades.
