This is misleading. The markets definitely move, they just don't show a massive spike in the charts.
Are people just banking on the assumption that it's going to go up higher than how much they have to pay back over time? What if that stops being true 5 years from now somehow, like another coin becomes dominant? Not saying it's likely, I do think Bitcoin will probably still be doing well in 5 years, but I'm not certain of it.
It's also kind of unclear what the interest is from that page. edit: apparently it's 8.5% https://mkr.tools/governance/stabilityfee
And one more thing, it's not technically an interest rate, it's a stability fee (which btw, gets burned, used to go back to stakers, but they removed that).
There are also centralized versions with blockfi if you prefer traditional loans.
If it dips below 150% the contract is automatically liquidated. Should give ample time for the creditor to get their money back.
Another option is, now that it's becoming clear to people that Bitcoin is here to stay, you can just borrow against it to spend (avoid taxes, hold on to the upside). Case in point, this person/group borrowed 300M$ with about 1B$ net worth[1].
1: https://defiexplore.com/cdp/8463