Which one is correct to say "Hello everyone": 大家好 or 你好大家?
While 大家 does mean 'everyone', it functions like the 2nd person plural pronoun ('you all') here. In *你好大家, there are two problems:

With 你 always being the 2nd person singular pronoun, the plurality is confusing.
Even if you say it like *你們好大家 to match the plurality, there is much redundancy. It is ungrammatical and nonsensical to add a subject (大家) after a topic–comment (你們好) sentence.

Much better if you say 大家好 or 你們好 instead. 大家 is friendlier than 你們.