It's a little bit more complicated than just not capping the maximum resolution. You have be be very carefully with your programming to properly handle the bigger amounts of data. Of course, increasing Minesweepers resolution to infinity is just a matter of increasing one variable in your code, but programming a modern graphics intensive game requires some sophisticated engineering that cannot just be substituted by raw processing power. It's basically the reason why 6 teraflops isn't a useful metric for your GPUs power. Often, data has to be transported between different parts of your program and also between various places on the chipset, so that different functions can work on this. At that stage, you can do lots of things very wrong. You might not be able to implement your algorithm straight forward, as this would result in the data being stuck in transport most of the time. Most of the time, there's no standard recipe to solve these problems as well. It comes down to the programmers skill and experience to come up with clever ways.

As I've said, in simple games (which include HL2 nowadays) you won't probably feel the effect of the "poor" programming, because their complexity is low enough, that raw power can push it to 4k anyway. But times have changed, modern shaders and rendering algorithms are so much more involved than back then. The difference is not just more flops, but much more involved maths. The basic phong shader is simple to understand, but modern physical based rendering? Not so much :-)

That's why we have dedicated hardware for graphics in the first place. CPU are a poor environment to implement graphics. Fast vector operations on huge amounts of data is a prime example for this. But it goes waaaay beyond that.

So yeah, you need to have know-how on both the hardware and the software development side, to be able to push modern graphics to the limit. If it where as easy as just increasing the number of cores on the GPUs we would see GPUs with 20 980ti chipsets on it, and call it a day.