Theia reuses some components (like the Monaco code editor), but the reason it looks and feels a lot like VSCode is mostly that they decided to copy it.
They explain the reasons for creating a VSCode clone here [1]. In short, they don't consider VSCode to be fully open source, and Theia wants the code to be both free and unencumbered by Microsoft's licensing and proprietary code, and also available to commercial vendors to use as the foundation for commercial products. That includes an open extension system that isn't owned by anyone, unlike Microsoft's marketplace.
Secondly, Theia is two things. One is an IDE positioned to compete with VSCode and every other IDE out there. The other is as an extensible platform to build IDEs, either open source or proprietary. This is exactly the same strategy as the Eclipse platform, except built in web tech.
This earlier HN discussion has a lot more discussion [2].
I think the Eclipse people could have communicated all of this a lot better. When you put a VSCode clone in front of developers, your first task needs to be to explain how it's not a fork, and why it exists. The Theia site's front page [3] doesn't even mention VSCode by name.
[1] https://blogs.eclipse.org/post/mike-milinkovich/eclipse-thei...