Because a lot of smart people are spending a lot of time, money, and effort on this. It's as simple as that. We could go into all sorts of details, like how increase in GPU capabilities will improve training capabilities, both in size and speed, or how GPU(/TPU) capabilities will improve, or how better techniques will make training on the same data set result in better models, or where other improvements will make better use of existing models or make them better or where we're seeing additions to training data sets and how that will improve models using existing techniques. But it really all boils down to a lot of smart people, some with a lot of money, that are personally invested (with time and money) in making them better.

That doesn't mean there isn't possibly a plateau somewhere but it's somewhere way off in the distance.