Why do we say "Ten people tested positive today"?
Shouldn’t it be "Ten people were tested positive today" as the lab technicians are testing the samples?
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It could be because the use of the passive voice for the second sentence reduces clarity slightly and takes emphasis off of the subject of the sentence. If someone says
Ten people tested positive today.
The emphasis is on the subject since the sentence is in the active voice. If someone says
Ten people were tested positive today.
The use of the passive voice has the effect of placing emphasis on the action of testing (since the sentence agent is missing), which may be more desirable in scientific or clinical writing to create the appearance of an objective, fact-based discourse.
I believe that sentences like "Ten people tested positive today." are more common in everyday speech because speakers wish to place emphasis on the people who were tested positive, not on the action of testing or the unspoken agent of the testing (the laboratory technicians). As Jason Bassford suggested, this has probably has more to do with the nuances of idiomatic usage rather than formal grammar rules.