Trading is a zero-sum game, but investing is not, and trading exists to make markets and snipe inefficiencies. The amount of money that can be made here can be surprising, until you consider that they're oiling the gears for manifold trillions of dollars in commerce and wealth. Some of that is gambling, but most of it isn't. Stuff like institutional hedgers and investment funds are huge sources of market activity.
It's is very hard to see how predicting short term market position unrolling by individual institutions is making a market or sniping inefficiency. That is an arms race that creates a minimum cost to enter to compete.
When success at sniping depends upon privileged network access, both human and technical, it's more a matter of profiting on an inefficiency you enforce upon everyone else.
I want to make an important note here: trading is not a zero-sum game. It generates value through the exact mechanisms that you noted, market making (liquidity) and inefficiency elimination (accuracy). Via the efficient market hypothesis, the value created by adding this asset liquidity and price accuracy finds its way back into the hands of the traders, often through real value increases in the assets going to traders with call positions.
This functions to keep demand for trading moving by making the average market return stay close to or above the market discount rate. So stock trading doesn't end up as zero-sum, but rather benefits parties at the market discount rate. If it didn't, no one would trade but those seeking to gamble, and there would be no notion of fundamental analysis in the market.
In reality, outside of an ideally efficient market, it's easy to intuitively suspect that the average return associated with trading is above the discount rate, and that more savvy investors obviously earn higher returns. This is more or less correct: the S&P averaged around 8.5% annual returns over the last few decades [1], and that only increases the farther you go back and lower the more recent your range's early bound is, corroborating the association between unideal market efficiency and high returns. Both "investing" and "trading" are less distinct in the stock market than in, say, VC, and so that 8.5% applies equally in aggregate.
But trading is most certainly not a zero-sum game. Like all market activity, it converges to producing value at the market discount rate in an efficient market. What that rate is is incredibly hard to say, but it can vaguely be associated with S&P returns, which have hovered around 5-6% in recent years on time-weighted rolling averages.
