Mostly for search engin indexing reasons. With a frameset each frame contains an individual html page. Without additional information for example robots text search engine spiders can not easily index the site correctly.
Reason 2. Someone may not use a frames compliant browser. 99.9% are but possibly some are not. It's easy for anyone to right-click and open a frame in a new window to circumvent your intended layout.
Reason 3. A visually impared person navigating a frames site may have more difficulty reading it using special software.