Because if you really become a Buddha, you can naturally speak various dialects and speak with one voice, just like Sakyamuni Buddha, and all beings will get it. Why doesn't he understand the living Buddha in Tibet and the language of China? Such a trivial matter can prove that he did not become a Buddha at once.
What's more, the living Buddha of later generations died without last words and was expressionless when he was born. It's all arranged by people and decided by lot, so it goes without saying.