Why do contacts degrade over time?

Our tears contain all kinds of stuff, like proteins, calcium , fats, lipids, mucous....ect...

The combination of all of those things plug up the matrix of the lens over time and no amount of cleaning can remove it. When the lenses are plugged up, there is no more oxygen transmission to the corneas.

Different materials have different life times , which is why some are daily, some are 2 week and some are good for a month. Some people who produce more of those ingredients than average in their tears find they can't get a full month out of monthly lenses comfortably and have to switch to 2 week lenses. Many others wear the monthly lenses for 2 months...they are much more durable materials than the 2 week or daily lenses , but not the best lenses for everyone.

All soft contacts wear out pretty quickly. They are from 38% to 66% water so they absorb a lot of stuff and a lot of stuff sticks to them. All contacts should probably be replaced at least every month. So why are there products on the market that claim that they last a year? Until the late 80s, contact lens manufacturers had not figured out how to make lenses cheaply enough that they could be replaced frequently enough. They sought and obtained FDA approval for their lenses to be replaced on a schedule that people could afford. There may have been some lenses designed since then that are so custom made that they are still not very affordable and have longer replacement schedules recommended. However most lenses that are marked annual replacement are 30 year old technology at a per lens price that is way higher than new technology.