Because you may want to implement things you care about, not necessarily the things the V8 developers care about.<p>Unless you mean &quot;why write your own engine instead of forking V8&quot;, in which case the answer might be that if you have the time to do it, it&#x27;s easier to become expert on something you created yourself rather than an existing large complex system.  And while you&#x27;re at it you can make sure the system you create is good at solving the problems you mean to solve, which the existing system may not be.<p>A concrete example of one of the design tradeoffs here: V8 doesn&#x27;t have an interpreter mode, just several levels of JIT.  Every single other browser JS engine does have an interpreter, because it turns out that for cold code (which is most code) that&#x27;s both faster and less memory-intensive than a JIT.  Not to mention allowing your engine to run on hardware for which you haven&#x27;t created a JIT yet.