Um, because scientists want to know if there is life beyond ours. They don't assert that aliens created life on earth though. Do you think aliens designed life on earth?
The issue in a nutshell is, what should be taught in science classrooms? The answer is science.
In science class, we study proven theory, gedanken, and new theories. You believe in evolution, you believe in a large, diverse universe, with many planets, you believe it is possible for higher life forms and lower life forms to have evolved elsewhere (you accuse Christians of being Earthnocentric in certain ways, including Creation), you believe scientific minds can legitimately believe aliens capable of communicating are worth millions spent on SETI... you question me about aliens designing life on Earth, thinking to entrap me, despite the fact that PROMINENT evolutionists have taught/do teach space seed theory, when you know I ALREADY BELIEVE AS A CHRISTIAN THAT AN ALIEN ENTITY INTENTIONALLY FASHIONED LIFE ON EARTH. Good one!
...You believe we should not even CONSIDER space seed in science classrooms because you accept NO possibilities beyond proven science, you would have therefore said "NO SETI!" just as surely you continue to preach "No praying toward God until we prove He exists!"
Nice triple standard, there... You want us 1) to spend money to shoot radio signals into space to contact presumed, non-proven entities, but you will 2) neither waste words to pray to God to possibly inherit eternal life and miss eternal Hell, nor do you 3) want our precious children to be exposed to the philosophical and scientific underpinnings to SETI.
THINK BIGGER, darling, to paraphrase Tom Hardy!