What. Are you saying maintainable code is specifically related to startups? I can accept companies as an answer (although there are other places to cut your teeth), but startups is a weird carveout.

Writing maintainable code is learned by writing large codebases. Working in an existing codebase doesn't teach you it, so most people working at large companies do not build the skill since they don't build many large new projects. Some do but most don't. But at startups you basically have to build a big new codebase.