Well, "because it just does" is a reasonable answer.
I recall that Test Cricket once didn't have a time limit and that all matches were played to a conclusion, nowdays this would play merry havoc with schedules. But all the cricket playing nations are a long way from each other (exception for ex-British India*, who weren't playing each other then as it was all one dominion) and when international cricket started the only way of travlling was by boat. If you've spent several months going to Australia by boat you'd want to play to a finish too.
Now with the five day limit the game may end up in a draw if they run out of time.
The ideal cricket experience is take the day off, go down to Lords, have it rain all day and have the game end in a draw without you seeing any play. Especially if the England team is bad and a draw is the best they are going to get.
* India-Pakistan is now the single highest stakes derby game in the world bar none.