So 1) space is super hostile. Not an ideal environment to operate anything in.

2. everything eventually needs maintenance and it's basically impossible to perform maintenance on satellites so think how long a complex device would function on earth without anybody touching it ever again.

Both of the above lead to 3) they engineer the living shit out of those things, and damn near everything has a "safety factor." Which, in short, means they take whatever they think the operating threshold is for a component and divide it by 2 at the very least when doing their math on what they think the component can withstand for any given attribute. This is a common thing in almost every product developed. Ever see a ladder that says rated for 250 lbs? Odds are it won't fail until you put 500-750lbs on it. But nobody says that number because you can't guarantee it won't fail just before 500 and you don't want a user to find out, so you say 250lbs.

What this means is NASA guarantees the satellite will run for 2 years, but it will likely run for longer than that. Plus, every good engineer operates under the rule of "under promise, over deliver." Tell your superiors it will definitely last for 2yrs, engineer it with safety factors to guarantee 2 years, and then over deliver when it ends up lasting for 2.5, or 3, or in this case, 5.