As many have stated, at the end of the regular season in any sport, there are always a number of teams that are statistically so close and there are so many variables that the "best" team cannot be determined.
You could vote.
You could let the computers decide.
You could have all the "potential best" teams engage in some sort of contest.
Wouldn't the winner of this contest be the best?
A round robin competition for all the "potentially best" teams would be the theoretical way but, of course, that is not practical in many instances.
So, playoffs are the practical solution. Perfect? Of course not. But, as they say, it is better than any other alternative.
And, if there is no way to ever determine which team is "best", isn't the term totally irrelevant?