Many reasons:

The champion can be crowned by the middle of the season
A driver can finish 2nd in EVERY race, another driver can win 2 races and DNF in all the others and still be champion.

There are a lot of permutations of this that will wreak havoc with naming a champion.

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You have your way. I have my way. As for the right way, the correct way, and the only way, it does not exist.