I read:
But is my impression correct, that media art isn't really sold, but more done for hire? Like, some institue comes along and tells someone: Please make us an impressive installation, and we pay you.
I think this is by no means a practice exclusive to media art, contemporary composers pay their rent by writing commissioned pieces, conceptual artists go on (slightly paid) residencies and produce documentation about their work. Even a lot of fine arts people that are regarded as highly successful are commisioned by museums to make a sculpture and probably make most of their living income by teaching or giving lectures.
This wouldn't be that much different from the way, some open source software developers earn their money. You gain reputation while working on, say, the Apache webserver and then someone, who needs to setup a customized webserver, asks you to do that, for money.
I guess these two things have more in common and are less different (at least on a economical level) then one might think at first. see http://www.paulgraham.com/hp.html
take care,
x