I'm fuzzy on the timeline, but there was a time where the emerging CSS wasn't up to the task of creating more complex layouts (no flexbox, not to speak of the grid). CSS Zen Garden was just there to show what could be done by that point.
Things have certainly gotten easier, but even back then CSS had things like pseudo-elements and display: table-cell. The main problem was Internet Explorer 6 holding everybody back so floats were the main way of laying things out, and those go back to CSS 1 (1996).
