Russian clubs tend to be very protective of their youth prospects, and that's true even when you are playing in Russia. The Russian league has foreigner limits so every club needs players of Russian nationality that they can play every week. And it can no longer simply be Russian second nationality, gained through living there. So even moreso than making leagues, Russian clubs have incentive to keep their prospects. Not to mention that many clubs in the top division are pretty financially healthy. Zenit is outright wealthy.