system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it still a challenge for some countries to build ICBMs while US/USSR could already do it in the 50's?
gpt
ICBM technology requires an extremely advanced research and development basis in aerospace engineering and rocket science. Which in turn requires strong institutions specializing in physics and mathematical research. 

But that strong research basis will only get you the theory and design because engineering a rocket requires that your country also have a manufacturing sector that includes the skillful production of precision instrumentation, specialized machinery, advanced application of metallurgy, and chemical processing for fuel and lubricants. Which, by the way, requires even more knowledge and research to implement because all those necessary requirements would take decades to develop.

But that's not all, guidance systems require that your country also have an advanced electronics industry capable of producing components that can withstand conditions normal electrical equipment would simply break down mid trajectory.

Now, you can shortcut this extremely complicated process by buying and importing all those things that are necessary for the development of ICBMs, which is what North Korea did. But that still doesn't take into account the expertise and infrastructure you need to test and produce a viable system. 

The ICBM still needs a warhead which has its own development process, but actually relatively easier than producing the delivery system. 

So the countries that developed ICBMs are countries that had the foundation prior to their goal of producing ICBMs. The United States, and the Soviet Union, already had a strong industrial base and scientific research community to lean on to develop their ICBMs, not to mention that they both were able to acquire expertise and know-how by importing former Nazi scientists and capturing the research the Nazi regime made in developing their own rocket technology. Germany was (and still is) a great scientific and industrial center. 

Many countries even developed and advanced economies, lack ALL the components required to develop ICBMs. Even developed countries that do have the necessary factors to develop ICBMs choose not do so because the allocation of resources toward the goal of producing an ICBM would be economically ruinous for their country and result in major hyperinflation and suffering. It would take years and the resources put into developing an ICBM would be resources wouldn't pay for a national healthcare system, public education, retirement payments, etc. So many countries simply ally with the US and sign treaties asking to be under the protection of their ICBM program, for example, NATO. And those countries also agree not to pursue technology like that under international treaties at the behest of the superpowers.