Because there are others that want to study the life on those planets. According to the Outer Space Treaty we have to act to prevent harmful contamination of another planet. "Harmful contamination" here has taken to include harm to the scientific experiments of other parties to the treaty.

To take an example, if you introduced Earth microbes to Mars and they flourished there, then from then on we'd never know if the life we found was from Mars. And no, there is no genetic test that can tell if life is Earth based or not. Not if it has a common origin. The Mars life could also be vulnerable to Earth life, simple example, if it is an earlier form of life, e.g. RNA only life - it would probably have no defences against the more modern advanced DNA based life. You can think up many similar scenarios.

So - it's unethical in the same way that introducing rabbits and rats and cane toads to an isolated island with unique indigenous species is unethical.

You might think it is different if there is no life there yet. But even then it is also an ethical quandry.  First, scientists are also very interested in how a planet like Earth behaves if it doesn't have life on it. We could learn a lot about exoplanets for instance. And may find out a lot about how easy it is for life to evolve and about early pre-life precursors.

Also, even if somehow there is no present scientific interest at all (unlikely) - well you are taking responsibility on yourself to transform an entire planet in an irreversible way. Perhaps this is something that advanced civilizations can do knowing the consequences. And then maybe they can nurture that world for millions of years into a desired final state, or to help it through its early stages of life.

But we have no experience of terraforming worlds. We don't even know what is likely to go wrong, never mind how to fix it. And we have less than 100 yeas of spaceflight. How can we think we are ready to take on the multi-million year responsibility of introducing life to a new planet?

See also

Our Ethical Responsibilities To Baby Terraformed Worlds - Like Parents