Why only seven feet?
Human fitness (survival chances x reproductive success) in relation to height follows a bell curve to some degree, as does height itself. Too short is seen as unmanly and unattractive, but true giants, i.e. people over 7 feet tall, often have health problems - bad joints, heart failure at a young age, etc (you may have noticed how many celebrity giants have died in their 30s or 40s in this way). So no, female preference for tall men isn't going to result in a race of 10 foot tall giants anytime soon.
There's also the fact that true outliers in any direction just seem freakish and weird to most people rather than attractive.