The reason Shi Hao could succeed was because he was the protagonist, possessing an aura of the leading character, born to tread this path.
Meng Tian Zheng was already incredibly monstrous. Without any assistance, he found his own way, and was hailed as the first person of the Nine Heavens and Ten Earths; he almost broke the record in this age where it's impossible to become an Immortal. But even such terrifying talent ended in failure, didn't it?
Did he think that these rabbit cubs from Liu Village could also succeed? It was nothing more than a case of the blind leading the blind - a clumsy imitation that could very well end up in heavy casualties.
The most critical issue was that Jiang Hui currently had no ability to protect them on this path! Otherwise, he would not have taken the risk to seize the Heavenly Seed and the Tree of All Paths!