It isn't low expectations. It is the implications of high demands. When they spec a project to last 5 flights, they are serious about that guarantee. There are so many redundancies built in the make sure that no matter the curve ball the planet throws at them, they will get their 5 flights out of it. That is on the worst case scenario side though. Then you factor in best case, and now you have things lasting years longer than their planed mission. This is how most of their projects end up lasting so much longer than planned, while also having almost every project last the minimum. Those projects minimums are all but a promise that they will do it, barring catastrophic events.

It is also more cost and time efficient. If there was a project that was going take 10 years and cost $10M with a 50% success rate and you were told 'for each extra year and $1M-$2M we can increase that success by another 10%' then you better believe, as a tax payer, I would be passed if they didn't spend the extra time and money to get it up to at least a 90% success chance.

Now just scale that to hundreds of millions and billions of dollars. Some of these projects are measured in decades and take people's entire careers. If you had a project that was 20 years of design, build, test, launch, and travel- wouldn't you want to do some work on its ensure success?