The difference is that there is a defined end-point, set by the actual perceived odds of the horse winning. The price would not jump up that high instantaneously, purely due to the volume of the market. It’s not like the stock market where worth is pretty arbitrary - the race is about to go off, and if you’ve backed at 8-1 about a horse that has been backed mostly at 7-2 and below then you’ve got value.
It’s true that it’s a zero-sum game, but the difference is that the majority of players are taking a permanent view, unlike the stock market where people predominantly buy to sell later. The fact that there are defined increments in price also helps - if you can back at 3.4 then the next price notch down is 3.35, and nothing can come between those notches.
Like I say, not everyone can do it, but it works for me.