Why cropping an 18 megapixel image isn't as good as an adequate telephoto

An 18 megapixel image has twice the data as an 9 megapixel image.

So does that mean a 135 mm telephoto and an 18 megapixel sensor can do as well as a 270mm telephoto and a 9 megapixel image?

No.

Why not?

Because when you crop the 18 megapixel image, and double the apparent size, you also double the image noise.