Why is it bad to wear contact lenses longer than advised?

Well the worst case scenario is corneal edema that can damage your eye to the point that you can not wear contacts at all. If you over-wear your contacts you are just asking for trouble down the road, perhaps not now but later on in life. Taking out your contacts every night is good, but are you cleaning them nightly? Contacts are a medical device and their use is regulated by the FDA. Your doctor should tell you the limits of the contacts and unfortunately if he/she tells you 2 weeks I wouldn't push it...Over-wearing contacts can lead to increased infections, microscopic complications related to contact lens wear discomfort and redness

Well it's a way for them to get you to buy more contact lenses, but that's only a fraction of the reason. One is that the contact lenses will get infected sooner or later and your eye can get infected, and it's not fun. You have to get this weird eye drop that will make your eye numb and then you have to take some special eye drops and then you can't wear contacts for a while. A good way to make contact lenses last longer and kill more germs is to use hydrogen peroxide solution. It's quite good.

Because they dont let enough oxygen get to the eyes and as a result blood vessels increase to allow more oxygen to the eye

they say to only wear them for so long, because they can get bacteria on them causing you to get an infection in your eye. i do not know how long it takes for your lenses to do this, but i think it is also a way to make more money. if you are taking them out at night and cleaning well enough, you should be okay for some time.

i did it so many times.. nothing happen yet