If the Saudis disinvested from the dollar and started selling oil in yuan, it would severely weaken US soft power around the world. At the same time, seizing the assets would remove a bargaining chip from potential peace talks. And those assets likely wouldn’t be enough for Ukraine to win anyway.
Turns out when you steal other countries’ assets (as with Venezuela), other countries stop trusting you with their money and start trusting other countries (like China) with it instead. If they wanna speedrun losing a second cold war, that’s the way to go.