From the credit reporting agency's perspective, they aren't looking at you as one holistic person that has a story involving all of your finances. They look at all of the separate aspects of your finances separately.

So from a matter of risk - your example of someone with more than 5 accounts paying the minimum each month is looked at in separate pieces. Only paying the minimum each month is an element of risk weighed much more heavily than the number of accounts they have.

Look at it from an apples to apples perspective. Person A has 10 accounts, with on time payments on all of them for 7 straight years, and a credit utilization of 5%. Person B has 9 accounts, with on on time payments on all of them for 7 straight years, and a credit utilization of 5%. Same amount of income, same average age of accounts, etc. All their computer cares about is that 1 extra account shows less risk in their ability to manage one additional payment every month.

Another way to look at it is that it's not that paying off the car makes you more risky, but that having an active account that is always paid on time (especially installment debt vs. revolving debt like a credit card) makes you less risky, so you get a boost while having that active installment loan that goes away once paid off.