Why did Timur not try to include the Golden Horde and Russia into his state, but only robbed and ravaged their lands?

Timur did not seek to conquer lands away from the southern branch of the Great Silk Road – he either plundered them (like India), or devastated them, like the Golden Horde, which controlled the northern branch of the Great Silk Road. Russia, according to Timur, was too poor to plunder, and hostile to the Horde, and therefore useful to Timur.