Waldo is hiding because his creator - the author/illustrator of the Where's Waldo books - decided it to be so. Who knows why. For this reason, to ask, "Why is Waldo hiding?" is essentially no different from asking, "Why does the Where's Waldo series exist in the first place?" Perhaps we can get Martin Handford himself to respond to this Challenge...

I still wouldn't milk/soda them and/or drink their soda. Instinctively it just feels weird. I mean...whose idea was it, however many thousands of years ago, to consume the bodily fluid from a cow? We are so accustomed to the concept nowadays. But just think about it for a second. It's really, really, really weird, and doesn't make any sense to me.