Maybe we don’t want to infect any life that is on that other planet, that earth life could take over that ecosystem like an invasive species? Astronomically unlikely
If you were to pick out any one microorganism and try to get it to grow on Mars without any support, you’re right that it would probably die off. If you were to take a pile of random dirt full of microorganisms and drop it on Mars, they would also probably all die off. But if you keep doing this a lot with dirt and rocks from many different environments on Earth, you may eventually find one that thrives.
There are organisms that carve out some tiny evolutionary niche until they have just the right conditions, and then explode. For example, Ideonella sakaiensis eats PET plastics. It was sitting around doing its thing for millions or billions of years, and then we gave it a place to thrive with all our plastic junk.
There are places on Earth that have some similarities to Mars. It’s quite possible something would survive there.