It's because LLM tools have design guidelines as a part of the system prompt which makes everything look the same unless you explicitly tell it otherwise.
To give an example that annoys me to no end, Google's Antigravity insists on making everything "anthropomorphic", which gets interpreted as overtly rounded corners, way too much padding everywhere, and sometimes even text gradients (huge no-no in design). So, unless you instruct it otherwise, every webpage it creates looks like a lame attempt at this: https://m3.material.io/
To give an example that annoys me to no end, Google's Antigravity insists on making everything "anthropomorphic", which gets interpreted as overtly rounded corners, way too much padding everywhere, and sometimes even text gradients (huge no-no in design). So, unless you instruct it otherwise, every webpage it creates looks like a lame attempt at this: https://m3.material.io/