Blissey: Seismic Toss is Blissey's most reliable attack doing a steady 100 HP damage against all Pokemon but Ghost types. It's base 75 special attack and base 5 attack aren't very good, so it's just better to go with the set damage.
Dusclops: Let's put it simple. Terrible offenses, so why not just go for the set damage? Its attacks will almost always be doing less than 100 HP to the opposing Pokemon.
May add more later.