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.

You have just restated the similarity I referred to. The ways they are different make them important enough to distinguish.

“Tabulate” doesn’t just mean organising anything by rows and columns, it means organising data for a particular purpose. And layout grids usually end up looking quite different to tables because although they have a broadly similar underlying structure, the purpose is quite different.