Your question's very weird honestly. It's completely unfocused and therefore really hard to answer satisfactory.

First, There are very many different proteins with different functions. They look nothing alike. So yes, some are stretchy or bendable, some are easy to manufacture. Others, not so much.

You learned DNA/RNA, those are like a prescription/recipe on how to form a protein. They're "just a strain of repeated units than can be read by ribosomes" (for simplicity's sake we stick to the highschool explanation here).

The ribosomes "read" the recipe and built the protein strand, which starts out as a long chain of amino acids. Each amino acid has its own unique characteristics like pH or sulpher atoms. When connected into a strand, those characteristics influence nearby amino acids to bend away or towards them. Polarity and Hydrogen and sulpher bridges shape the strand into a 3D structure. This is the "why": interactions make it that way. That kinda answers your question as well.

The last answer is "because it works". Proteins which are useless and cost energy to make or have a cool function but cannot be broken down afterwards affect the cell negatively. Efficient cells on average do better than inefficient ones, so good traits survive in the long run (yes, again, I simplify). Structures that work well are conserved throughout evolution, hence why we can make whole trees based on similarity. Which is also a reason why they're structural (and even more so than based on sequence similarity alone)