I really don’t understand the months in advance planners. Even if I WANTED to be that busy I genuinely don’t have enough stuff I could fill the next 20 weekends with. I have a good group of friends, a full time job, a dog, hobbies…but I’ve never understood how people get so booked up and what with
We both volunteer. It sometimes involves taking young people away for the weekend, and sometimes for a week or 10 days, and very often for individual days at weekends. Apart from the actual time on the event, none of these thing just 'happen'. There is a huge amount of planning goes into each and every event including things like First Aid Training and Safeguarding training, Risk assessments and all the logistics , food buying, prepping equipment, getting all the paperwork in place.
Other people commit to running drama groups, Park runs, orchestras, Martial Arts training, swimming galas, etc etc etc.
Surely classes/lessons are only once or twice a week and even if you have a set weekend commitment that’s only going to be a few hours on one of the days?
.....per child
I don't understand the lack of flexibility around those fixed things
Really ? You don't see that there is a level of commitment to the rest of the Team or Band or whatever else people are relying on each other in ? 
I don’t schedule in when I’m going to clean or do laundry, I just do it when I have the inclination. It seems bonkers to me to plan in stuff like that.
I agree, nor do I, but there does come a time when someone has to do it as you run out of food, or clean clothes, or your car insurance is due, or your car actually breaks down, or you need to make appts for things like dentist, optician, smear test, hair cut, and so forth
Likewise asking someone for a coffee in January - I’ll 100% forget about it before January comes around and probably end up double booking myself. Unless something is happening within the next 2 weeks I will forget
So what I am hearing here is, your "friends" mean so little to you, you can't use the seconds it would take to add that in to your diary.
I don’t think the planners and the spontaneous people will ever understand each other tbh.
Well, as I have said before - I love it when people pop in unexpectedly, or I bump into someone on a walk and they say 'have you time to get a coffee?'. Really makes my day to have an unexpected chinwag when I have time. Sometimes, at the end of a meeting, people say 'Let's go and get a drink' and again, I like that. So I like spontaneity but I also know that people are busy, and if you really want to see them, it makes sense to make firm arrangements. I don't think you have to be one or the other at all.