Why Java compiler as distributed as executable and not as JVM bytecode?

Question

In the answer Platform independence in Java ByteCode, it is said that Java Compiler is same across all the platforms. I do understand that.

My question is that why

Answer 1

javac.exe (on my installation, JDK 1.8 on Windows x64) is about 15K in size. This isn't the full compiler. The compiler itself really is written in Java, and javac.exe is just a launcher, effectively. This is true of many of the tools that come with Java - it would be a pain to have to run something like:

java -cp path/to/javac.jar java.tools.Javac -cp path/to/your/libraries Foo.java

for example.