They still have to transship through another country, because they don't have access to the ocean - and it's not as if they're surrounded by stable or western friendly countries. Plus Kazakhstan traditionally has quite strong relations with Russia, though it should be noted Kazakhstan is supporting Ukraine.

Regardless, the world's largest enrichment facilities are in Russia. They are expensive and take many years to build - the US only has one enrichment facility itself. For the foreseeable future, Russia is a major player with nuclear power generation.