Banks offer a stable or floating rate for your investment/deposit every 3/6/12 months.
Yes, but it's not enough. Offering me ~3% in returns when there's a nearly 10% inflation in my country does only make me poorer.
Bitcoin on the other hand, doesn't guarantee you a certain percentage of performance, but the possibility to rise in the future.
And nobody guarantees you that the value of the dollar, the euro, the pound will continue as is. It might rise, it might fall. But, nobody knows for sure. Same goes for bitcoin. However, during the last 10 years, bitcoin has risen by more than 180% in every year, on average, while the fiat currency I use has a cumulative inflation of
18.58%.