Generally, software engineers have difficulty with system design interviews (SDIs) for three primary reasons:

• SDIs are unstructured, where candidates are asked to take on an open-ended design problem that doesn't have a standard solution.
• Candidates lack experience in developing complex and large-scale systems.
• Candidates did not spend enough time preparing for SDIs.

SDIs are similar to coding interviews in that candidates who don't prepare well tend to do poorly, particularly at high-profile companies like Google, Facebook, Amazon, and Microsoft