There are two factors: extraction and boil-off. A flavor add gives time to extract almost all hop oils, but the lighter ones are lost. Dry hopping only extracts the very lightest oils, but there's no boil-off.
Dry hopping is NOT a substitute for flavor/aroma and flameout adds. I learned this the hard way, years ago. Just like chocolate sprinkles on your cupcake do not make up for a lack of frosting. You can boil hops 5 minutes and add the tea to your fermenter. Read about "hop tea" for more info. "French press" is another good search term.
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