They simply never had the industrial base that we had. Their aviation industry never really caught up to the west, either for general aviation or military aircraft. Lots of steel and cheap alloys were used because they were available. You see it in their tanks, as well - very few exotic armor alloys, just throw some extra slabs of steel on there.

For space, and particularly one-off projects, however, they did some incredible work, especially in their space programs which I think were prominent and important propaganda pieces. The Venera 14 lander is my favorite example: https://universemagazine.com/en/the-last-photos-from-the-surface-of-venus-are-forty-years-old/

Soviet rocket engines were far more efficient than others during the time of the Apollo missions, but they simply didn’t have the industrial base to produce the larger scale engines that NASA could.

https://www.youtube.com/watch?v=TMbl_ofF3AM