I did explain it. The plane rolls because if the treadmill moves 300 MPH in the opposite direction it isn't enough drag to stop the plane from taking off.

Thrust > Drag = plane moves and flies.

I'll drag up this example...

OK lets say you're on roller skates and standing on a moving sidewalk. I'm not on the sidewalk. I hand you a tow rope and walk the opposite direction of the sidewalk. Do you move? yes

Let's say we quadruple the speed of the sidewalk and I still walk in the opposite direction. Do you still move? Yes

How much drag does the moving sidewalk affect me? The answer is very little.

The plane's engines overcome the minimal amount of drag the conveyor offers and the plane takes off.

The only way the conveyor stops the plane is if it can produce enough speed which in turn will produce enough drag to counter the engine's thrust. It however isn't matching the planes speed at that point but greatly exceeding it. It isn't going 50 MPH in the opposite direction. I would have to go much faster than that to stop a plane that needs to get up to 50 MPH to take off.

That simple enough?