Although the fusion system allowed Companion Beasts to advance to a higher level, there were too many uncertain factors involved. The failure rate was too high, and it might change some important characteristics of a Companion Beast.
After all, not all Companion Beasts had Jinxes Master, so it was fine for him to occasionally try his luck or experiment with large numbers of Companion Beasts of the same type.
Zhou Wen didn't dare randomly fuse rare Companion Beasts. If he was unlucky, he would be flabbergasted if they were destroyed.
The Samsara Stone was different. Companion Beasts that used the Samsara Stone not only had an Evolvable attribute, but they could also be renurtured. Their evolution direction was easily controlled, allowing him to forge the Companion Beast he wanted.