I used to think like this - until I had kids! They usually demand constant or near-constant attention, and you give as much as you can, but especially once they're into the toddler bracket and they don't literally need you to hold them/feed them/do everything for them like a newborn, you have to take little breaks where you can.
If I'm out in town all day with a toddler, on our feet, talking about all the things we see, explaining why we can't go into every single shop, playing a lot of I Spy, counting things at any opportunity, finding toilets, chasing pigeons etc... by the time we get to the train home we might've been going non-stop for hours. Eventually you just need to be able to say to them "please stop asking me questions/pointing things out/trying to stuff crisps into my mouth and entertain yourself for ten minutes until we get to our stop."
To an onlooker that might seem like a parent who just ignores their child. But you're seeing them for the duration of your train journey and no more. You don't know what else they've been doing or will be doing, you don't know how well or otherwise they're coping with their day/week/life.
Ultimately one of the biggest lessons parenting has taught me is not to criticise other people's parenting if it isn't actively dangerous - and hope they'll afford the same respect to me and mine.