MisAdventure I agree with you, in principle, but I do think that one's relationship with a permanently single child is closer than with a married child.
I noticed this with my parents and my unmarried sister and I am aware of it with my daughter. I think it mainly arises from them not having someone to use as a sounding board at home when life goes pear-shaped or if they need help, in situations where a partner might provide moral support.
As a mature independent adult there is a proper space between us and DD, but when she was woken by a horrible noise a few weeks ago and saw some animal (a badger, she discovered later) attacking (and killing and eating) 'her' hedgehogs. Her response was to ring us the next day and tell us all about it, whereas for DH it would have been a family tragedy, dealt with, with wife and children with us being told next time we spoke or when he posted it on Facebook.
When she had her accident, we were the only people she could turn to and as we were retired, we could rise to the occasion. If we had been still working, the situation would have been different, she would have been longer in hospital and when discharged been, at least partially, dependent on carers and hospital transport.