I like to think it's out of concern for you that they are trying to dissuade you from a solitary lifestyle. You like to know that your loved ones are taken care of. And, if you're the kind of person who enjoys or needs support from other humans, it's really hard to understand that someone else isn't interested in that. Usually when someone is "a loner" its because they're a shitty person, or they have shitty social skills, or some other such thing. Less often, the solitary individual just prefers solitude. I've seen it come up on this sub a handful of times.

Like I said... I would LIKE to think that's where its coming from. But your mother's snarky comment about grandchildren is pretty... off-putting. I think all that can be done, if you care to preserve your relationship with your parents (for your sake or for theirs) is to be as non-confrontational as possible and perhaps execute things like "I can see that we disagree on this issue. I really am happy and fulfilled living my life for myself. Lets discuss something else." when it comes up in conversation. Avoid, avoid, avoid talking about it.

They likely won't ever "get it", so maybe the best course of action is to just not discuss it with them.