Also if you repay your loan early is somehow bad for your credit score.

1. The tradeline doesn't disappear from your credit report when you pay it off. It continues to benefit your average age of accounts for up to ten years (note that credit score estimates like Credit Karma do not work this way, and stop considering the loan the instant it's closed, which is not the way it works at the three credit bureaus—more info on the differences between Credit Karma's system and your actual credit score here).
2. It's trivial to have and maintain a good credit score with a revolving credit line (e.g. credit card) you're using and paying every month; installment loans are temporary by definition, and considering that loans with 0% interest essentially don't exist, they are not the way to go about building your credit score; they're what you use your good credit score to get as good a rate as possible.

With regular credit card use only, my credit score is well over 750 (and 750+ is top-tier from the perspective of basically 100% of lenders). And the last installment loan I had (car purchase over a decade ago), I coincidentally DID pay off early. Also, my average credit age, just checked, is 7y 9mo, less than the ten years mentioned above.

To scam citizens out of yields while minimising the chance of nonperforming loans?

Credit scores can only benefit good borrowers. Without them, everyone gets treated the same as people who have never borrowed, and lenders are obviously going to err on the side of caution (read: higher interest rates) when lending to someone who's a big question mark. But with credit scores, lenders can know who the ones who do make their payments regularly are, in other words, who it's least risky to lend to, which leads to lower interest rates.

In short, without credit scores, everyone gets shitty rates. With them, only shitty borrowers get shitty rates.

To reiterate, the bottom line is that you don't need to pay a single penny of interest to have a superb credit score. Just use a credit card and don't borrow more than you can pay off every month, same way you'd be limited if you were spending cash on the spot each time. That's literally all it takes.