This is just my opinion but I think it's like this:
For casual friends, old work colleagues, etc, when they hear their old friend has dementia, they just write that person off as if they were dead. As far as they are concerned, the person they knew has now gone and that's that. For us, we know the person we love is 'going' but they are still there somewhere and we're still attached with all our memories. Casual friends just don't have that ongoing connection so to them it's all over and there's no point them having further contact.
Of course, it's also much easier for them to take this view than to actually look further/try harder so you find out who your real friends are. It also shows a shocking disregard for the other half of the couple because unfortunately they get dropped too.
Perhaps the truth is that no one really likes being around people with dementia. Carers/spouses just don't have the choice.
Maybe I shouldn't have said that - I hope I haven't caused offence.