They don't. 'Trees' is pronounced /tri:z/ - no native speaker would say /tri:s/.
There are two possibilities here:
1. You are misinterpreting what you hear. You might be hearing a very light 'z' which you perceive as an 's'.
2. These people are not native English speakers. People whose first language is Spanish or German, for example, often struggle to pronounce voiced final consonants and so mispronounce 'trees' as /tri:s/.
Would you like to send us a link to a clip where you have found people pronouncing such words in this way?