The very short answer is that imperialism requires very specific economic systems and incentives. Those systems are not going to occur in socialist States because socialist States develop different economic systems than capitalism because the profit motive is absent, which impacts short term and long term economic development plans in many significant ways. For an extreme example look at Juche’s emphasis on self reliant socialism within an internationalist socialist order. They cannot do imperialism because all of their economic planning is built around a stable self sufficient economy. An extractivist economy isn’t just something you can graft on, it has to be a central part of an economy to make economic sense.

For an example of socialism not being imperialist when it has the opportunity to, you can look at China forgiving loans. It doesn’t do so out if the charity of its heart, it does so because it is incentivized to because damaging other nations self determination through financial coercion actively harms its project. It wants strong neighbors with close economic ties, it doesn’t want to suck the marrow out of their bones because that is destructive to China in the long term, and socialism is able to plan in the long term unlike capitalism which has to be more short term oriented because of the way its incentives function.

Imperialism is actually a very costly affair (in many cases it costs the home country and only benefits specific lobbyists within that country) compared to mutual cooperation and always rebounds on empire, it only happens because of market failures that do not happen under socialism.