Comment by adontz

Because you can have something usable instead of dreams. If you want to have completely independent code eventually, then replace modules one by one, while having a working product. Like Mozilla did with Servo. They had C++ codebase they wanted to replace with Rust codebase. They did not create entire browser from scratch, but replaced C++ with Rust gradually. While one figures what HTML and CSS layout should look like, no need to have no JavaScript engine and basically unusable browser. Use existing JavaScript engine, replace it later when/if project has enough resources.