Why is this the case?
Because, astrobiology is inherently multidisciplinary. To search for aliens requires a grasp of, at least, astronomy, biology, geology, and planetary science. Undergraduate courses in astrobiology need to cover elements of all these different disciplines, and postgraduate and postdoctoral astrobiology researchers likewise need to be familiar with most or all of them. By forcing multiple scientific disciplines to interact, astrobiology is stimulating a partial reunification of the sciences. It is helping to move 21st-century science away from the extreme specialisation of today and back towards the more interdisciplinary outlook that prevailed in earlier times. By producing broadminded scientists, familiar with multiple aspects of the natural world, the study of astrobiology therefore enriches the whole scientific enterprise.
It is from this cross-fertilisation of ideas that future discoveries may be expected, and such discoveries will comprise a permanent legacy of astrobiology, even if they do not include the discovery of alien life.
It is also important to recognise that astrobiology is an incredibly open-ended endeavour.
Searching for life in the universe takes us from extreme environments on Earth, to the plains and sub-surface of Mars, the icy satellites of the giant planets, and on to the all-but-infinite variety of planets orbiting other stars.