And WTF would Russia keep their assets in the west? Maybe ask yourself that.
Uhmmmm... because that's how the world's financial and trade system works?
Have you ever wondered why the US doesn't seem to hold any foreign currencies as FX reserves, but everybody else has to? All the US Fed holds is some gold bars, and in an amount that's not enough to cover US imports for even a few weeks. The gold is there, mostly, to provide comfort to foreigners that at least there is something backing their greenback.
The reason that countries like China, Russia, etc sit on huge piles of dollars and Euros is because our citizens and companies need them to execute their trades.
How does this work? Ivan's company wants to buy a new piece of machinery for their factory, which is manufactured by a company in Malaysia. The Malaysians invoice him an amount in dollars, because nobody in Malaysia wants to receive Rubles, and its difficult for us to buy Malaysian Ringgit in Moscow. So Ivan goes to his bank, and sells rubles in exchange for dollars. His bank would have typically received these dollars from the Central Bank, again, in exchange for rubles. Once Ivan has his dollars, he can send it to his Malaysian vendor and then receive his goods.
America doesn't have to keep piles of Yuan or Rubles, trillions of dollars worth, because its citizens and its businesses can do business with most of the world just by sending and receiving dollars. This is one aspect of what a French President once called America's exorbitant privilege.
So back to why those assets could be confiscated? The reason is that money is mostly digital now, and if you are dealing with dollars, then you are always dealing with US based custodian banks that act as clearinghouses for your operations.
Believe me, Russia does not want to keep its money in the West any longer. Neither does China, or India, or Brazil, or any government with a functioning brain. Your geniuses in the West blew up the best racket that ever existed, over what... Ukraine?!