We have some of the most intelligent people in the world working on extracting as much wealth as possible from high volume trading. For someone to gain money in the market someone most lose. Imagine if we had these minds working on solving real world problems that would actually benefit society. Just like gambling, trading is zero sum.
That largely depends on what asset is being traded. If this asset is entirely speculative, i.e. it doesn't have any use value in real life (read, it is a shitcoin), then it is definitely a zero-sum game. However, if the asset traded has a certain application in real life which requires this asset to be actually employed for something other than trading (for example, Bitcoin as a value transfer device), then it can't be a zero-sum game as some people would be buying it for purposes which have nothing to do with the "buy low, sell high" mantra (the speculation epitome)
Hope this helps with better understanding of trading as a zero-sum game