Components needs to have MTBF of many, many years, or the combination of many components would get a lousy MTBF.

But even real products needs very long MTBF or you would constantly need to service them.

Think about it. An MTBF of 300000 hours (34 years) that I mentioned as an example for a HDD. What does that mean? If stretching it (for a theoretical product that doesn't have wear and tear) there would be a 50% probability that the disk would fail before 34 years and 50% probability that it would take longer. But that means that a couple of percent of the disks installed would fail every year, so a company installing 100 machines (with one disk each) would probably need to repair 2 or 3 machines every year because of a failed HDD. They would also need to service a couple of computers each year because of a failed PSU. And ac couple because of a failed motherboard. Or failed memory. Or failed fans. Or failed...

Now think about producing a modem. Let's say you sell 1000 modems each year. How many of these modems can you accept comes back for warranty repair? If you think MTBF 1 year is ok, then you would be very busy repairing modems since 50% of your modems would fail faster than 1 year and 50% would fail after more than 1 year. How many can you afford to repair for free (since we are talking about warranty) and still make a profit?

It's only for products with very short usability time span that you can afford a short MTBF. A normal pen or pencil isn't expected to live so long, and people aren't likely to send them in for warranty repair.

But before you sell something, you must think about your potential warranty costs. The sell price can't just cover the costs for R&D, marketing and manufacturing. It must also cover the potential warranty costs while leaving enough money that you make a profit.