For some non-parents, it's not obvious that being in the same room (or building) as the kid(s) is nowhere near enough. Small children in particular can only briefly occupy themselves; the rest of the time, they need your attention - and they will demand it, loudly.

That last part was something I didn't internalize until I became a father myself. There's no "you do your stuff, and I'll be looking to make sure you won't hurt yourself" with small kids. They won't just do their stuff, they want you to do it with them. On top of that, once they learn to move (even just by rolling around the room), it's always 5 seconds from "everything is fine" to "the kid is trying to lick the electrical outlet".