I suspect most people think that both are caused by old age, rather than that deafness causes (or accelerates) dementia. It's not like there's a well known group of congenitally deaf people who get dementia young to inspire a causal connection.

My mum started noticeably losing her hearing after her dementia symptoms got quite bad. She may well have lost her hearing sooner and we just didn't notice, just as in retrospect there were huge warning signs of dementia well before the diagnosis and we just didn't realise what we were observing.