Why does the mRNA transcribed from exon 10 not have a start and stop codon?

Exons are spliced out of the pre-mRNA.

DNA > pre-mRNA > exons/introns

Basically, the exon is cut out from the middle of the pre-mRNA - thus the start/stop codons are left with the introns.

I do not remember how many exons are contained within the CFTR however, assuming that exon 10 is not the final exon of the coding sequence (or otherwise known as the open reading frame) it would not need to have the stop codon. Given that there are numerous exons (9 according to your question) that are spliced into the CFTR transcript that are well 5' of this exon it is not likely to carry the start codon for translation either. There are numerous exons which only encode untranslated sequence for mRNAs for a number of genes and they will not harbor either a start or a stop codon.