Off topic:Why is ribosomal protein taken as a reference for highly expressed genes

Question

[rehalabar08]

I am having problems in assimilating the concept of ribosomal protein being highly expressed and use of these set as a reference one to calculate codon adaptation index value. Why do we take ribosomal protein as highly expressed genes? After transcription we have mRNA, tRNA and rRNA (rRNA along with ribosomal protein form subunit). May be this question of mine is silly but where do these ribosomal protein come from prior to translation and yeah some say they are in the machinery and conserved so if that is the case why do we check the expression level. I am so confused and unable to get csatisfying answer to it.