Because Mars isn't carpeted with life. It is generally thought that if life does indeed exist on Mars, it's likely to be at the poles, in the ice, and/or under

Because Mars isn't carpeted with life. It is generally thought that if life does indeed exist on Mars, it's likely to be at the poles, in the ice, and/or under the surface where most of the water is. Life on Earth, on the other hand, carpets the Earth and is almost impossible to completely kill. For example, most space probes are sterilized for forward planetary protection, but they can't be completely sterilized, so they're instead sterilized to different standards. Here's the standard under COSPAR for Mars, > then the entire landed system shall be sterilized to the following levels: a surface biological burden level of 30 spores (see 5.3.2.4) and a total (surface, mated, and encapsulated) bioburden level of 1.5x10^-4 spores (see 5.3.2.4). 5.3.2.4 states, > 5.3.2.4 The 30 spore figure takes into account the occurrence of hardy organisms with respect to the sterilization modality. This specification assumes attainment of PP Category IVa surface cleanliness, followed by at least a four order-of-magnitude reduction in viable organisms. Verification of bioburden level is based on presterilization bioburden assessment and knowledge of reduction factor for the sterilization modality. Life on Earth has survived, amongst other things, - On the outside of spacecraft, for several years. - Beyond the photosphere in places like geothermal vents or the movile cave, https://en.wikipedia.org/wiki/Movile_Cave - Hyperacidic and hypersaline lakes - In volcanoes - Anoxic lakes Given that there are - almost certainly - more varieties of life in more niches on Earth, and that these species often co-exist with humans. If any large-scale settlement of Mars occur, then human by-products like sewage, and other by-products produced through agriculture, are likely to cause sub-surface contamination. All it takes is one place for the microbe laden water to seep through into the subsurface water of Mars, and we may end up killing off life on Mars. More specifically, the worry isn't that they interact with each other, but that our life will out compete the martians for the same resources. Backwards contamination is less of a concern, at least right now, as we may not be that biocompatible and it is extremely unlikely that the Martians will have the right proteins etc to get past our cell membranes and make us sick. The most plausible disease scenario is the one discussed in the Expanse, where an alien organism finds its way into the vitreous humor of the eyes and starts growing there, causing blindness.