The difference is the incentive to improve, and actual present rate of improvement, for models like this is far higher than it is for jetpacks. (That and certain intrinsic features at least suggest the route to improvement is roughly "more of the same," vs "needs massive unknown breakthrough".)

Trillions of dollars are not being invested in making jet packs any better

Your comparison is incorrect

While true, it's the speed of improvement that gives the statement gravity.

> Personal jetpacks are the worst they’ll ever be

Have they become better over the past 20 years?

It's also just wrong. Plenty of things get worse.

Not really. E.g. clay tablets, physical books will not be meaningfully better.