Hint: Think about align-items' own default value, and what that default value specifically does to an item that hasn't been given its own explicit cross-axis size.

align-items' default value is specifically stretch, not something more neutral like flex-start, which means unless a flex item has an explicit height, in a row-direction container, or width, in a column-direction container, set on it, or unless the container's own align-items has been explicitly overridden to something else, flexbox automatically stretches that item to fill the container's entire available size along the cross axis. This default stretching behavior is a deliberate flexbox design choice, intended to produce visually consistent, equal-height, or equal-width, items by default in many common layouts, like a row of cards that should all match the tallest one's height automatically, without needing any extra CSS to explicitly force that matching. It's precisely this default, rather than any accidental behavior, that's responsible for flex items appearing to fill the full cross-axis size when no explicit size was given, and switching align-items to a value like flex-start instead would let each item size itself based purely on its own content instead.