Why can a plane lift off into ground effect near the runway but still need to accelerate before climbing away?

Answer: Ground effect lowers drag

Ground effect lowers drag ✓ — Right. Close to the ground, wingtip vortices and downwash are reduced, so induced drag falls and the airplane can feel unusually willing to float. But that bargain fades as it climbs out of ground effect, so it needs more speed to handle the returning drag. This is why soft-field technique can use a low liftoff but not a low-speed climb-away.

Runway air is denser — No. Air near the runway can be warmer, cooler, denser, or thinner depending on weather and altitude; density is not the special mechanism called ground effect. The key change is geometric: the ground interferes with the wing's vortex pattern. That reduces induced drag close to the surface, and the benefit weakens as height increases.

Engines add extra lift — No. Engine thrust helps acceleration and climb, but the near-runway surprise here is not that the engines suddenly create extra wing lift. Ground effect changes the drag cost of making lift. That means the airplane can lift off in a temporarily easier aerodynamic neighborhood, then need more speed when that neighborhood disappears.