I’ve asked myself this question (in a different context) and have concluded that, in general, developers are incredibly resistant to learning and using new languages.
For example: if they weren’t, then server-side JavaScript would never have become popular.
You make it sound like a personal psychological failing.
In real life, using the same language has business benefits: re-use of the same libraries, no duplicated business logic that needs to be used on both client/server, no need for training to the new language, no need for mental context switch due to the language change when working on the client or server part (aside from the essential domain knowledge of client vs server), less/common tooling infrastructure, and so on...