Tables are bad for layout because:

A) Screen readers (for visually impaired people) have trouble knowing that the 75th <td> 3 miles down in your code is actually related to the 6th <td> tag in your code. While the visual structure may look coherant to you when viewed in a browser, it barely ever reads the same to a screen reader.

B) Tables were intended to layout tabular data... thats it. You wouldn't build your house with Play Dough just because it is easier to use would you?

C) Take it from someone who used tables for a while before seeing the light; designing and updating a site with <div>s and CSS is a million times easier than with tables. Tables are actually harder to use to layout a site...