It's on holiday and away from home that siblings close in age come into their own in a day to day basis. At home the kids all have their own friends. Away from home they hang out together.
Mind you even in that context some siblings fight more and age gaps have to be very small for it to work. My eldest is 5 years older than my youngest (3 kids) and they are not peers - she likes "looking after" him, helping him, teaching him stuff but he doesn't always want that. They get on great and I don't think they have ever argued or fought because they are too far apart to have that bickery sibling vibe, but that also means that they are not exactly playmates (though they are company for one another and like being together).
Middle one has more of a peer relationship with both siblings but even the 3 year gap from him to dc3 means only some games work together. DC2 is big, strong and athletic and dc3 is also sporty but a bit on the skinny side and average height for his age - this means that despite the 3 year gap they are more like 5 years apart when it comes to dc2s favourite outdoor pass-times and still not much use as "ready made playmates".
My eldest two who are barely 2 years apart were absolutely inseperable until eldest was about nine, and absolutely were playmates and besties (they used to tell strangers they were twins, as they have hovered at roughly the same height since they were 4 and 6). That was lovely, but they've grown apart since eldest (the only girl) approached puberty, started secondary etc.
I like having a house full of kids and it makes it easier as a parent to go on holiday or for days out when they have siblings to play with, but it's only for a few short years that they're playmates, and even then only with the right mix of personalities and ages.
I hated my closest in age, same sex sibling growing up but I think that had a lot to do with incompatible personalities and how we were parented, especially the fact that I was always expected to take responsibility for her happiness and social life and to be the one rising above and turning the other cheek in arguments, disagreements and fights because of being older.
Onlies need thoughtful parenting but so do siblings.