They're built to last and with high quality.

The 2 years of mission time just means a 99% chance to be fully functional for the full 2 years. After that it's just a matter of individual parts failing until full contact loss. So after 2 years it can still do everything it could at launch but at year 4 it's still doing some stuff just maybe not everything.

The whole 2 year but really 5 year thing is also very helpful for funding. It looks good if they can say "yeah the last mission you paid for was a 2 year mission that lasted 5"

PS: this is just my understanding of it, please correct me if I'm wrong