Because we have telescopes and refractors that can clearly see other spherical planets.
The planets are not the earth.
Your argument is an analogy in favor of roundity. It is a favorite "argument," in support of the earth's spherical form. Astronomer John Wrottesley Wallace once poised to the Flat Earth Society "that as all the heavenly bodies are worlds, and visibly round, may, not the earth be so necessarily, seeing that it is one of the same category?"
This is only seemingly plausible. In reality it is a piece of self-deception. It must first be proved that the lights we see in the sky are worlds. To do this, or to make it even remotely possible that they are so, it must first be proved that they are millions of miles distant from the earth, and from each other, and are hundreds or thousands of miles in diameter. By plane trigonometry, in special connection with carefully measured base lines, Samuel Birley Robotham has demonstrated and placed beyond all power of doubt that the sun, moon, and stars are all within a distance of a few thousand miles from the surface of the earth. Therefore they are very small objects. Therefore not worlds. And therefore, from analogy, offers no logical reason or pretext for concluding that the earth is a sphere.