A lot of friendships are based on shared lifestyle, children change lifestyles even between friends that start families around the same time.
Children use up time and energy and there's a lot less left over to spend on friends.
Some families get little opportunity for child-free time.
Sometimes children have used up all your social energy so there's nothing else left, and you just want peace and rest.
Children have different interests that affect what kinds of plan you can make. Mine are at their best outdoors where it's appropriate to burn off energy. Sitting quietly is not their forte, neither is entertaining themselves with colouring and reading (thanks dyslexia and dyspraxia) so keeping them placid and civilised without resorting to tablets can be a minefield. My oldest has outgrown soft play and is very prone to sensory stimulation which is a problem at lots of venues.
My best pre-child friendship involves friends with very similar, compatible children, so our social life involves lots of walks/ Nat Trust/ collecting sticks. Other friendships are harder for keeping children mutually happy and the children have little in common. Now my DCs are getting older, some friendships are resurfacing. One friend and I are now able to go hill walking again and our DCs are able to happily tag along. We've also mastered travelling halfway to a mutually accessible parkrun and carving out 35 glorious minutes of running time together before heading home to our busy lives as before that we struggled with being 1hr+ from eachother and spending a day together.
Some friendships can handle one half having children, but the logistics of two sets of children with different needs will be too much.
The best friendships will still be there when you're ready for each other.