Comment by sussmannbaka

They don't! Layout grids are less about the rows and columns and more about the lines separating them (which is why those get a lot of attention in CSS grid). Take a look at how layout grids are used in design and you will quickly find examples that are extremely inconvenient to realize with HTML tables. I'm sure it can be done and I'm sure some poor email marketing dev had to, but the result would be entirely static and not able to reflow.