WIY, inertia is the resistance to movement. You mean momentum.
Uneeq, The bus does not have to keep pushing you. You accelerated together with the bus, since you were on the floor when it accelerated. The bus itself needs constant energy to keep moving because it has the wind and ground friction slowing it down. If not for those factors, as long as the bus is traveling on a straight line it would be able to keep on going.
There is nothing holding you back from moving so you keep on moving. If, however, the bus turns or slows down and you are not sitting down you will indeed ‘find yourself another seat.’
This can be observed in space. Once an object in space started moving there is nothing to slow it down and it keeps on moving. That’s how the satellites keep moving. They don’t have fuel tanks. They get one push in the beginning and they keep on moving.