We used to have capital gains indexed in the UK. I've never seen an inflation allowance for interest though. I guess it would be too expensive?
It's factually easier to say - if your earned interest is less than X(absolute value), then you don't even have to report it.
US tax system is MUCH more complicated, than UK's... and UK's system is fairly complex.
Interestingly, the current administration proposed to index capital gains taxes to inflation to address that very paradox. Unsurprisingly, their proposal would have just been a giveaway to the wealthy.[0]
[0] https://itep.org/why-trump-administrations-plan-to-index-cap...