There's a range of factors that could create a gulf between detecting a viable virus in the laboratory and a real-world risk of infection. For degradation, the environment is likely to be much more tightly controlled in the laboratory (low UV, reduced airflow, ect.). For exposure, a hand briefly touching a surface is very different from doing an intensive swab. For viability, the virus may be encapsulated in the fomite in such a way that it doesn't release back into the body in a viable form.