In the context of Mahayana Buddhism, it was considered that because one must have juki (endorsement) by Buddhas to attain Buddhahood based on their guidance, the way to become a Buddha is to be born in Buddha lands.
何故仏国土に往生する事が、成仏の方法となるかというと、成仏には、仏の導きと仏による成仏への保証(授記)がなければならないからである。