I think you missed a key detail. The collateral for taking out a loan earns staking rewards that get distributed to the depositors. The amount of money staking rewards are worth can fluctuate too depending on the state of the project / cryptocurrency ecosystem.
Yes, but most of them have a business plan that doesn't involve taking a loss, once they've reached a certain scale. And while they're taking that "loss" it's not usually due to revenues being below the cost of goods sold, it's because they're spending all profits + some investor money to grow. Many (though not all) can simply stop growing and be instantly profitable. Businesses that sell you $10 for $5 tend to fail as soon as they run out of capital, because they've got no path to profitability.