Final answer:
Pre-mRNAs are capped to protect them from degradation and initiate translation, while tRNAs and rRNAs undergo different modifications that serve their specific functions in protein synthesis.
Step-by-step explanation:
The reason why pre-mRNAs are capped but tRNAs and rRNAs aren't is because pre-mRNAs require protection from degradation and recognition by factors involved in translation. The 5' cap, a 7-methylguanosine nucleotide, is added to the 5' end of the growing pre-mRNA transcript, preventing degradation and helping initiate translation by ribosomes. On the other hand, tRNAs and rRNAs undergo different types of modifications, such as cleavage, methylation, and chemical conversions, but not capping, as they serve different functions in protein synthesis.