Some cells have a lot of content, and the height is set to be larger; for some cells, the content is less, and the height is set to be smaller. Excel can do it, why can’t airtable do it?
Its (likely) a design decision. Spreadsheet software like Excel or Google Sheets typically allow flexible row heights, while some low-level relational database software like Airtable or Google Tables do not.
why is it designed like that? Since low-level relational?
Not a clue. I suspect its because databases typically hold more content in a particular cell than spreadsheets do, and since Airtable serves the entire table at once without pagination, it is potentially more beneficial for those with an immense amount of data to crop rows to am maximum height.