'Too much time has passed. I suppose I'm wondering if I might have regrets but if someone who can drive over in an hour hasn't come by, or invited you, for six years, it's over isn't it. They have zero interest in me and my life.'
OP, not sure you are being realistic. In my experience - I have 3 kids, grown up now - parents who look after their own kids (ie who don't have live-in nannies or au pairs) simply cannot 'drive over in an hour', even at a weekend. Even when kids are old enough to go out by themselves, there's just too much to do - housework; work-related work - and you're either physically tired and/or mentally jaded.
Partner & I have some old friends - also parents - who live well within walking distance, but we're both too knackered/brain dead to organise anything so hardly ever see each other, which is sad. But if one of them rang to say, hey it's been a while, shall I pop over tonight, we'd be like, oh no look at the state of the place, we're planning to get sorted for work tomorrow, get the kids in bed by 9 (which means 9.45), watch Call The Midwife and crash out ourselves. And so are they!
Also, kids don't go away at 16, 18 or 21 anymore. Eldest is 31, working from home, earning good money but because of this country's ridiculously broken housing situation, finding it difficult to move out.
Weekends don't make much difference; I do shift work so might be working anyway, and ofc weekends are taken up with cleaning, washing, shopping. You're right, the gap between parents & non-parents is massive.
For myself, I have zero friends from school or uni (and was far too diffident to imagine anyone would want to stay in touch). I have an honorary cousin I've known since about 14 but she lives 140 miles away. I have 2 good workmates left; used to be 4 but have lost 2 of them & I don't know why. I never socialised with any of them outside of work/pub after work, so there's no particular reason why we couldn't carry on as before, they've just gone off me or have too much else going on, I guess.
Sorry, this is not very encouraging but I'm afraid I don't think deep friendship is very compatible with today's busy, not-enough-time-energy-or-money society.