That’s a reasonable idea. I can imagine a ribosome directly binding to DNA to translate the DNA codons directly into protein. We can’t really tell why that didn’t happen, as much of the early development of life occurred far in the past and left no evidence of the steps in the process; we must infer the process from observing modern biochemistry. However, I can speculate that much of the regulation of gene expression occurs at the RNA level, with variations in the stability of RNA, translational repression by mRNA, inhibition by natural antisense transcripts, etc. Perhaps the RNA intermediate was required in early life to provide a mechanism for regulation of gene expression.