Why write a new language rather than expand on an existing one or an existing ecosystem? Maybe I'm not the intended audience, but personally, I struggle to thin

Why write a new language rather than expand on an existing one or an existing ecosystem? Maybe I'm not the intended audience, but personally, I struggle to think of any reasons to use a new language except to get access to an ecosystem of packages I wouldn't be able to use otherwise. Something like TypeScript, for example, really caught my attention because of how well it integrated into the existing JS ecosystem, adding sanity while still not sacrificing interoperability.