The Frieza Saga bomb was from Namek and some of the surrounding planets. Almost all life on Namek had already been wiped out.
As for asking for energy vs taking it, it could be that Earth was too far for him to take it directly. It's possible that the stronger Goku is, the greater the range he can take energy from, but he was really far from Earth while making the Buu bomb.
Chances are, the planet of the Kais is far away from any living planets, and was unable to forcibly take enough energy to defeat Buu, so he needed energy sent.