If you try to render tables with millions of cells the browser does a really poor job and the performance is abysmal. The only solution when you need to render that many cells is to virtualize the table and only have the visible cells (plus some buffer) actually in the DOM at a time. That plus weird restrictions browsers put on certain table elements (looking at you thead) that prevent them from being "sticky" headers means that the developer is left with absolutely positioned divs as the only solution. Blame browser vendors for not providing a native way to present tabular data with more than a few hundred thousand rows without causing performance issues.
there's table-layout:fixed that makes rendering of large tables much faster.
I'd argue that if you have so many rows that DOM can't handle, humans won't either. Then you need search, filtering, data exports, not JS attaching a faked scrollbar to millions of rows.
