Why do pros wait in the fountain when their nexus is being destroyed?

Im pretty new to league and in all the pro games ive watched the players who are still alive when the enemy team is hitting their nexus just wait to lose in the fountain. Is this them trying to preserve their KDA, or just sportsmanship as there is no way left to defend?

In my mind with so much at stake when winning, there isnt a reason to accept defeat before the game is over, even if the chance to defend the nexus is 0.00001%, its still worth a try.

Ty for any responses, tried googling but couldnt find anything.