The answer is that currency conversion is not as simple as you suggest. The dollar to ruble exchange you suggest would only be available on the black market, since dollars from banks are scarce. The black market is likely to have a higher ruble to dollar exchange.

An economist might also observe that if you deposit money in a Russian bank the interest rate is rewarding you for taking the risk that the bank will collapse and you will lose all your money.

There is also the risk that if you deposit money in a bank you will be prohibited from withdrawing it. The money will be seized to pay for Putin's war.