We could have sent seeds of life (say a few ounces of microbes) along with every planetary exploration craft. Why we haven't done that so far?
Planetary protection is the principle that on the contrary to your proposal, we should take steps to avoid any contamination of planetary bodies with life from Earth.
There are various cited reasons for this, one of the most important being that since we don't yet know whether extraterrestrial life exists on these planets, if we accidentally or deliberately introduce terrestrial life that could confound future experiments and make it much harder to determine whether extraterrestrial life is present.
As noted, there is a strong desire to NOT send terrestrial 'anything' to other planets.
They spend a LOT of money and time properly sterilizing space craft that land or might crash land into other planets. Now you might say, this vehicle will be in interplanetary space for 1-2 years in transit, surely nothing can survive.
Yet amusingly odd stuff has been found to survive on the surface of the ISS for years since launch. (Something about sea plankton DNA?).
Elon Musk has told the story many many times, that his initial plan before starting SpaceX was to build a contained greenhouse as part of a Mars lander, so that on landing it would add water and starting growing green things on the red planet.
Once he realized the issue was not enthusiasm but rather launch costs, he abandoned that approach and instead focused on launch costs via SpaceX. (Which he has succeeded at spectacularly so far! Everyone is forced to drop launch costs, Chinese do not think they can compete. Ariane 6 is designed specifically to try to compete. If the landing succeeds on any of the launches in the coming years, and they are able to refly first stages it is likely his success will become only greater).