Not to take away from the great work of NASA engineers but EOL is kind of a misleading term when it comes to space science missions. At lot of mission end times are based off of funding and the lowest level science goals of the mission. While those are being completed they gather a case for continuing the mission and usually get it. Of course the main cost is getting the stuff there in the first place so why not add a relatively small amount to make it last?

Not to take away from that but I suspect that there are a lot of good reasons to underpromise and overdeliver. I also wonder to what degree budgeting has something to do with it. Budget for a shorter lifetime given that you can usually end up extending missions if the equipment is still operational.