The requirements of the Adult Dependent Relative visa are that the dependent (parent in this case) must
(a) need help in performing basic personal functions, such as bathing and washing, etc, AND
(b) that such help is either unavailable or unaffordable in their country of habitual residence.
Given that the cost of living is normally cheaper outside the UK, the second part usually fails. The logic is that if you can afford to pay for their care in the UK, you can almost certainly afford to pay for it in their country of habitual residence.
Home Office stats suggest that 94% of applications fail on application, with the success rate going up to only ~20% after appeals (i.e.. even after appeals, 4 out of every 5 ADR applications fail).
And once an ADR application has been submitted, the chances of the parent ever getting a visit visa are next to nil, as they have already shown an interest in residing in the UK rather than just visiting.
I had written on the reasons for these policies in
another thread.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.