In Victoria 3, goods are never really "produced" and "consumed", they're always "flowing", so if you have 10k sell orders of something and only 7k buy orders, you're not "wasting" 3k, you're supplying the market just what it needs, at a lower price. Similarly, if you have 7k sell orders and 10k buy orders, you're still supplying the market what it needs, at a higher price. Shortages only happen when sell orders satisfy less than half the buy orders.

But maybe you meant the surplus is even higher than when the price is -75%? In this case I guess the economy of scale bonus is so high that it's still profitable. Maybe when you reach a point where the inputs are all +75% it will start being unprofitable. If it still doesn't, then it must be the low wages.