I don't recall that there was ever anything inherently wrong with using tables for layout, except that it was a misuse of tables so we were told it wasn't "semantic". Thus you had years of people asking on forums how to emulate tables using a mess of floating divs until flexbox/grid came around. In retrospect, tables are also clearly incompatible with phone screens, but that wasn't really a problem at the time.
One, it made the code unreadable and impossible to maintain properly, especially since most of those table where generated straight out of photoshop or whatever.