I keep hearing stories of nurses whose jobs become unfeasible (40 day on shifts and such) but they are getting 3x pay rates and other perks like signup bonuses

I keep hearing stories of nurses whose jobs become unfeasible (40 day on shifts and such) but they are getting 3x pay rates and other perks like signup bonuses to work for "traveling nursing" outfits. From the very little I've read on it seems like the employment agencies are negotiating better than the nurses unions. It appears that theres a perceived need to have an intermediate agency between the hospital and its staff now but I don't get why that is.