I think Ashleigh's attempts at meeting people and making friends reminds Hubert of earlier, happier days, cracking the fiction he's concocted for his life and prompting his subconscious to push him back out into the world. His troubles finding new friends his own age (or even reconnecting with his previous friends) are unfortunately all too common.
In my experience, many adults lack opportunities for making friends because life gets busy and revolves around work and other responsibilities--not like when you're in school and meet people your own age constantly. It takes more conscious effort to find or create community, and not everyone wants or has energy to do that. Or time. Even with my long-time friends, we don't get together very often at all because our lives are just in different directions, so it takes planning and effort and calendar coordination. If it weren't for meeting my kids' friends' parents, my social circle really wouldn't expand at all any more.
Once your kids are grown, as Hubert's are, and especially once you're retired, as Hubert is, those opportunities for making friends shrink to practically nothing unless you actively seek community at a senior center or church/mosque/synagogue, etc. And as was mentioned in a training session I attended yesterday, the advent of air conditioning triggered the reduction in neighbors interacting, so it's not in the least surprising that Hubert doesn't know any of his neighbors and therefore has no community watching out for him or each other.
In my neighborhood, I've been very deliberate in meeting and interacting with my nearest neighbors, and even then it's hard to maintain consistent connection with anyone who doesn't also take kids to or from the school bus stop. Hooray for nice weather when we're more likely to be outside in our yards or taking walks! An unexpected side effect of the pandemic was that more people were closer to home and outside, so we actually interacted with more neighbors more often the past couple of years, though that's slowing as things have opened back up and people have returned to old habits and patterns of behavior.
I love how their group in the book brings awareness to the problem of loneliness and the need for community. Even for introverts--just because you need to recharge after interacting with people doesn't mean that you don't still need those positive interactions, those community connections.