Compiling code at runtime may improve performance, but it doesn't give the kind of guarantees that static typing gives you.
Static type checking proves that certain invariants hold at runtime and hence that certain defects are not possible. If a program can manipulate itself at runtime in arbitrary ways (not just reflect on itself in a read-only fashion), those proofs become invalid.
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Compiling code at runtime may improve performance, but it doesn't give the kind of guarantees that static typing gives you.
Static type checking proves that certain invariants hold at runtime and hence that certain defects are not possible. If a program can manipulate itself at runtime in arbitrary ways (not just reflect on itself in a read-only fashion), those proofs become invalid.