Nobody forced them to do that. Some people just do things out of passion. I actually prefer those. It's not all about money. They probably wouldn't say 'no' to a paycheck from Bethesda but on the other hand they never promised them that or signed any contracts so I don't see a reason they should be expected to do that.
On another note, we have an example of Malukah who was hired to perform as bard in ESO so that's pretty cool and well deserved.