I think it's a combination of individuals who want the money, but not the work that goes into this profession, and people who are jaded by the constant grinding hard work of caring for unappreciative violent miserable people.
That does not for one moment excuse or condone such behaviour - merely explains it.
My youngest daughter works with senile patients in a care home. She has the patience of several saints, and always smiles and ignores their abuse, and avoids their violence when possible - and her home is a good one with no evidence of abuse of lacking of care.
But of course, there are plenty of people in homes who are just not that lucky, which is tragic. If nothing else, the elderly deserve the dignity of care and respect in their final years.