Question about Spaceship propulsion in the absence of inertia

It is a scientifically accepted fact that, because there is no inertia in space, if you propel something in a particular direction, at a particular speed in space it will continue going in that direction at the same speed.

For example if an astronaut fires his thrusters in space and then turns them off he'll keep moving at the same constant speed; he won't slow down.

So if a starship began its journey in outer space all you should need to do is push the space rocket at a speed approaching the speed of light once for a very brief moment, say hypothetically a second, and the starship would keep travelling at that same speed in the same direction without fuel.

And so I am puzzled when Scientists say that interstellar space flight would require a lot of energy. It would only require a lot of energy in a very short burst to send the ship in one direction but once the ship starts moving it wouldn't require any energy to keep it moving.

Can someone please explain to me why scientists think interstellar travel, besides that initial burst of energy, would consume a lot of energy?

I understand that to propel anything near to the speed of light would take an immense ammount of energy but they would only be required to generate that energy for a brief moment and then never require energy again for the entire journey except to stop/change direction or return home