Why stock selection is difficult?

I am new to investing, and almost all the books, articles and YT videos say that for new comers the safest strategy is to invest in index funds or ETFs. Turns out picking a stock is not an easy task. And I wonder why is that? What makes picking a stock difficult? (Is it because you can't process a lot of data, or because you don't understand macro economics, or because your don't understand the product a company produces or something else).

And another question is, are there any people who are consistently successful at selecting stocks? I mean is this something one can learn over time or this will be always hard because of xyz?