Many reasons. Your vision might be lowered due to uncorected need for eyeglasses but there is a long list of things which might cause a lowered vision. Some of these are progressive and need treatment. The inheritance of eyeglasses need is fuzzy and so you could have this even though mom and dad don't. Best to see you ophthalmologist to sort this out.
Many reasons. There are many reasons you may not be able to see 20/20 without correction. It is not entirely dependent on genetics. In fact, most simple refractive errors are due to individual factors not genetic. Environmental exposure, history of illness, other current medical conditions, and many unknown causes affect vision, be it simple refractive errors like near-sightedness or not.