I understand exactly what you mean OP. I think what it boils down to is that they are ultimately not alone. You walk on yr own, holiday alone, evenings alone. I am a lone parent, only one DC left at home now but they are late teens. I, like you , do everything above alone as everyone i know is also often "very busy". I am outgoing like you so, when i ask my friends if they would like to meet up i get the same kind of responses as you. What i am really trying to say to my friends is hey, i haven't had any grown up company for a while, and apart from work , have had no down time /conversation at all for months. Lets have a gin and pass some hours away. People often say if you are feeling alone, reach out to people. I have only ever reached out to people twice in my entire life and both times nothing. I read then that , if you over share/burden/say how lonely you are, that is not great either and puts pressure on others. Damned if you do damed if you don't.
I am happily single too, have my own home, good job, will never remarry or share my home again. Like going on dates when i want to. Like you, i just want to meet friends for good company, conversation and laughs. Always have. Made time for my friends always, even when i had young children and working and they were not and child free. I love my friends and enjoy their company. They are intelligent funny people.
I have no problems , nothing to complain about, so not like i am going to bring anyone down. I think people who are not " alone" and by that i mean , have people they go home to, go to Costa with, walk the dog with, go on holiday with , forget , what it is do these things alone all the time. When they are "busy", they really mean they just cannot be bothered as they have all they need and are not lonely basically.
I am of a view that friends should not be picked up and maintained just when it suits your circumstance. I think your friends OP are "busy" in the sense that they are not alone. I suspect should any of your friends find themselves in a position when they have to on holiday alone, live alone, go for coffee alone and walk the dog daily alone, basically do life alone, day after day, you may find that they are not as "busy" as they used to be!
I think OP, the answer here is to broaden your friendship group. Join clubs/societies/walking groups/fun dog training that sort of thing, to expose you to more people. More single people, women and men who want to meet up, go out and do things. People who are looking for a social life.
I do not think you are asking too much of your friends. A few hours of their time every few months is nothing. I have found that friends only realise that you may have needed a bit of company, once they find themselves alone. Even then , their new found enthusiasm for meeting up is self serving and only lasts until they meet the next person. Expand on your friendship group but equally, i would not personally be so available to those current friends who have left you alone all this time , especially should their circumstances change. I have let a lot of friendships " fade" now as they only seem to be in touch when they need something.