Whilst there are many vegan versions of things, I think it is the taste that is the issue.
Cheese is probably the best example. I have yet to find any vegan cheese that tastes even vaguely like some of my favourite cheeses. Mature Cheddar is the most obvious example, but I also used to use a lot of Comte, Jarlsberg, Capricorn Goat's Brie just to name one or two examples. I make homemade cashew nut cheese and can change the flavour by using different probiotics and by changing how long I age the cheese, but the favours are not the same. It also really does not melt the same way even when making 'melting' versions using Xantham gum etc.
I do now have a new book to try to experiment more with - not that I don't already do a lot of that. It is "The Non-Dairy Evolution Cookbook" by Skye Michael Conroy (ISBN 1499590423) and I am hoping to be able to come up with a good version of buttermilk for my Irish soda bread which I miss.
But I do agree that there are few reasons, if any, for not being a dietary vegan if the argument is simply 'what can't be replaced' and just because I miss it. (Incidentally, bacon has never been a 'thing' for me. I never really was that in to it.)