It's not as simple as that. Often, this is the only form of care that is funded, and the parents can't cope on their own. This is a societal problem, that we view disabled people as a drain and inconvenience, so we won't fund decent services.
Yes some parents did lock them up and forget as they were instructed by doctors to do, there was also so much willful deceit on the part of administration and people were threatened and harmed if they might tell their families what happened. Families were lied to and discouraged from visiting, told it was upsetting to the inmates (as they call themselves) and they were lied to and told their families didn't care about them. Many others were "dumped" there by CAS. So much of the "burden" mindset is embedded with ableism has yet to be undone by us as a community and society. We have so much to learn from disabled people about how to live interdependently.
Often back in the fifties that's where the doctors suggested parents put their children there many didn't realize that their children wouldn't be getting the care that was promised