Some people simply make their predictions without any specific reason. Cannot always trust the words of crypto experts which is why I feel that our own research and predictions are what matter at the end of the day. John Mcafee has been making some pretty optimistic predictions about bitcoin in recent times and I do agree with him in some ways. I do feel that bitcoin has the potential to touch 100k in the next 2 years, but 500k is way too optimistic.
Here's the thing, there is no such thing as a 'bitcoin expert' these people barely know anything about Bitcoin itself, the currency. They may know a tad bit about buying and selling Bitcoin, if they've made the money themselves or if they have just been studying it -- this doesn't make them an expert in the least.
These people are making these claims to make the market more bullish then it is and to make themselves more money. It's all bullshit though.