I found out that these interest rates are adjusted "by hand" by particular people, so they are going to have a certain amount of lag time. If shares become hard to borrow, expect a week's delay before the interest rates go up to attract more lenders.
I can imagine that they are adjusted by hand, but why would that take a week? The amount of money to be made by the lenders and the brokerage is amazing. Here are the latest Fidelity rates (they've been unchanged for several weeks):
Lend TSLA: 4.25%
Borrow TSLA: 7.000% ("estimated" - I assume "estimated" because they don't know what it will be over time)
Lend SCTY: 45%
Borrow SCTY: 67.500% ("estimated")
They report 0 shares available to short for both stocks.
A few more months like this and my SCTY could drop to $0 and I'd still come out ahead. Insane.