It matters what your profit margin is.
If you're making $700K/week, but it's taking you $30M in sales to make it, that's not that great (in this game, in reality that'd be exceptional), and the bank isn't going to loan you money.
The guys that manage huge loans early in the game put high-density planes on high-density routes with as many C/F seats as they can afford to, and the plane makes loads of cash. Then the bank is willing to loan them lots of money to finance the aggressive expansion.
Cash on hand also plays a role. The bank is much more willing to loan you money if you have lots of cash on hand than if you have little to none. So, basically, the rich get richer and the poor stay poor. There's a balance to strike between waiting a little while longer for the cash to come in and the amount of loan you'll get -- sometime I wait an extra real-life day if I think what I give up by waiting makes it worth it for the added loan I'll get because of having more cash on hand, making sure I look at my upcoming maintenances to make sure my margin will still stay high while I wait.