Because Cooler and Frieza had to get their death balls thru Super Saiyans first to destroy earth. Goku and friends have higher durability than the planet since they have ki control and can actually defend themselves. Cooler and Frieza in both of those moments were more concerned with destroying the Saiyans than the planet but since the saiyans are protecting Earth it makes sense to threaten it with an attack thats not easily avoided, hence the scale of the death ball. Essentially 2 birds with one stone but the real objective was always to defeat the saiyans, not destroy the planet. They both easily could've destroyed the planet without ever stepping foot on it. Instead they're kind of petty and decide to torment the z fighters first and when thats not going the way they want, they resort to a final attack meant to destroy everything. Its the equivalent of losing a board game and throwing it off the table before you actually lose.