> How come we don't expect people to understand that having a kid is a lot of work, something that will inherently set their life back, and price that into their decision of whether to have a kid?

Specifically because people understand this is why we need to accommodate people who have children. If having children starts really setting you back because it's not accommodated in the slightest, all the ambitious, driven, and responsible people will decide that they can't afford to have children. If it gets bad enough, it becomes so common that the fertility rate of your entire country falls down.

The other reason is a business oriented one: a very large portion of the population intends to have children. It would be rather strange to exclude them all from your employment pool.

> Can't we rephrase that as, someone who has a matter in their life they need to spend hours a day taking care of will have less hours for other things?

Now a question for you. Would you want to live in a world where the only way to survive was to dedicate 100% of your life to your job? No relationships, no hobbies, no time for things like exercise. Just your job.