By design. Having a slightly positive inflation rate gives you a cushion against deflation, which tends to cause market crashes. A currency I can be confident will lose a couple percent yearly is more useful than one that might be worth wildly different amounts year to year.

If you scroll down a bit on this page, you can see the massive inflation/deflation spikes in ~10yr cycles that existed prior to central banking.

https://www.in2013dollars.com/