The design lifetime is treated as a minimum acceptable value; a vehicle which was designed to last 10 years but has a critical component fail at 9.5 would be considered a failure, for instance. This means that the average lifespan of the vehicle gets pushed out a lot further to ensure it meets its goals.
With that being said, it's not uncommon for space vehicles to reach end-of-life for reasons other than a system failure - one common one being that a satellite or space probe runs out of propellant. Since the underlying mechanism there is predictable, rather than a random failure, there's much less margin needed.
Part of this is cultural, part of it is political: nobody wants a failed mission, it's better for the image of the agency and the involved politician to spend a bit more money and underestimate the lifetime. Higher chance of success, and nobody complains if the mission can be extended afterwards.
Eg, you need an industrial road with a 5-year lifespan over a swamp. To meet this minimum you actually have to build a bridge, which when built to industry standards, might start at lifespans of 20-30 yrs.
Space is a bit different because of budgeting for ongoing operations, so you frontload the cap-x, knowing that asking for addl op-x funds later to extend the program will seem like a no-brainer deal.
Plus sometimes it's as simple as: if you design something to statistically survive space launch, it results in something that is overdesigned to just sit in orbit for years (given that it survives that initial launch).
It's similar to human lifespan statistics- if you get over the historical infant mortality hump, every adult seems 'overdesigned' compared to the historical expected lifespan.
If something is built to last 10 years, it makes it likelier that it can survive another 10.
Don't forget that once upon a time, commercial-owned things lasted a lot longer too. Unfortunately the mba's took over and planned obsolescence became a thing.