Because I can do way more meaningful design exploration and iteration if I am not constantly running into a tool's limitations. I work at a fast paced startup where my prototyping rapidly iterates into production and the vast majority of developers I have ever worked with don't really know CSS. If I want to implement something actually complex in layout it would be SO MUCH FASTER if I could show the devs how to do it in CSS correctly in the design tool. AND it would let me better test and explore how the complex layout interacts with real data and real users. Figma prototypes are terrible.
Figma is a great tool for 90% of basic and boring design. A lot of product design is not just basic and boring, and a lot of stuff I need simply cannot be reproduced in Figma. So yes I do just write the code directly, but that doesn't let me explore those complicated layouts and iterate on them visually the same way I could if it was HTML/CSS in a Figma-like design canvas.
Figma is a great tool for 90% of basic and boring design. A lot of product design is not just basic and boring, and a lot of stuff I need simply cannot be reproduced in Figma. So yes I do just write the code directly, but that doesn't let me explore those complicated layouts and iterate on them visually the same way I could if it was HTML/CSS in a Figma-like design canvas.