There are four ways to lay out your site.
Originally there was just HTML. You could align your text to the right, left or center. Some elements had a default alignment. Web pages were all pretty boring but that was what was available.
A little later frames came along. You could divide up your web page into separate windows or frames. Now you could retain one section for a menu area on the left side of your page. You could have a consistent header section on every page. Currently frames are discouraged for several reasons.
Around the same time, some one figured out you could insert a table, make it the size of the whole page, and format your site that way. While this was quite clever, it was never the intention of the original html authors to have tables used for layout.
The current and correct way to lay out your page is with CSS- Cascading Style Sheets. CSS has several advantages:
So most sites must be using CSS now?
Surprisingly, NO. If you review 10 sites randomly you will probably find 8 or 9 use tables.
So most new sites must use CSS?
Surprisingly, NO. Again about 80% of newer sites are built with tables.
So what is going on?
Even though CSS specifications have been around since 1998, the browsers did not adopt them right away. However as of 2000 or whenever IE 5.5 came out there was full browser support. In the years from 1996 or so until 2000 a lot of web designers got very use to using tables. Now it is a bad habit they will not give up. They even 'justify' continuing to use tables for several reasons. But I 'justify' not going on a diet because I like my ice cream. I am not kidding anyone.
Another reason is that many web designers do not hand code. They crank out web pages with software, e.g. Dreamweaver, and many in fact just resell templates. The web pages created with WYSIWYG software and prepackaged templates are often table based.
Some will argue that table designs look better. I would refer you to csszengardens.com. Those pages are awesome.
I will grant that tables do a better job in some cases namely footer sections and liquid designs. But with a little effort CSS can accomplish the same.