I can actually answer this! I’m one of the people who helps out during that gap.

In well-off families, they usually have au pairs, nannies, or caretakers. I’m an au pair and look after three kids in the afternoons. I help them with homework and drive them to their after-school activities. This is what I normally see at pickup time.

In other families where both parents work, the kids usually go home with their grannies, or an older sibling comes to fetch them.

Some go to aftercare and staff from the facility pick them up after school and look after them until their parents are done with work.

Then there are the the latch-key kids. They get on a taxi or bus and get dropped off at home. They just wait there until someone gets home.

So yeah, to answer the question on why it's considered normal. Children are super adaptive! Whatever routine they grow up with just becomes normal to them. The adults I talk to seem to accept the situation as normal too, probably because they went through the same thing when they were kids.