Well, they don't - To clarify, only one seat one the market, the peg perego primo viaggio infant seat, expires in 5 years. Most of the rest have 6, 7, or 8 year expirations, and at least one has 9. Also, some expire exactly a certain number of years from date of manufacture, and some expire at the end of the 6th year, etc. Each manufacturer has it's own rules.
There are a number of issues that go into expiration dates, and one is that the plastic and the fibers on the harness straps degrade over time. Manufacturers perform tests on the seats and also the harnesses, including tests such as the effect of sunlight, water, juice, heat and cold on the seats.
Another important reason for expiration is simply that as technology improves, and we learn more about car safety, seats become better. Even a very top of the line seat from 7 or 8 years ago might be woefully deficient in terms of todays standards. Sort of like how there are still cars of the road with no frontal airbags...and now the newest advances are side airbags, and electronic stability control. We now have 5 point harnesses, higher weight limits, side impact protection, EPs and EPP energy-absorbing foam, LATCH tether straps, etc - all things that are relatively new. Things like overhead shields, 3-point harnesses, 20-lb rearfacing limits and shield boosters are a thing of the past!!!
A marathon that is 5 years old has one more year of life on it. Enjoy! And then, you can still sell the cover if you want - some covers can fetch a pretty penny if they are rare in good condition, but even common ones in good condition sell because lots of people with marathons like to have more than 1 cover.
Now, or course, seats do not disintegrate instantly on the day they expire, but we have no idea for how long past the expiration they continue to be safe. In a true hardship emergency situation, would I be okay using aseat a week, or even a month past it's expiration? Sure. But that would only be while drastic measures were taken to procure a new, safe seat.