Why do I cycle? I'm not sure there is a 'why'.
I was taught as a child. It was seen by my parents (and many others) as a thing children ought to be taught; like spelling, drawing, cooking, raking leaves, playing the piano and saying please and thank you.
It got me to the swimming pool and to cubs and (later) to school and scouts and friends' houses.
As with cooking, it has never occured to me to stop.
As with all the activities in my first paragraph, it has seemed a sensible thing to teach to my own children. So far it seems to have benefitted them as much as it did me.
There are many families today in which (even where money and space are plentiful) children are not taught to cycle. I am not sure why this is. It seems a missed opportunity, but I'm sure there are other and equally valid skills I haven't taught my kids.