I can provide some general reasons why some scientists and astronomers might find it implausible that aliens would crash-land on Earth frequently:
Vast distances: The universe is immense, and the distances between stars and galaxies are enormous. Interstellar travel would likely require advanced technology and immense resources. The chances of alien spacecraft frequently reaching Earth and crashing by chance alone may be considered statistically unlikely.
Survival of advanced civilizations: If extraterrestrial civilizations have developed the capability for interstellar travel, it is reasonable to assume they possess advanced technology and knowledge. Such civilizations would likely have sophisticated navigation systems and protocols to avoid collisions or crashes. Therefore, the idea of frequent crashes may seem improbable.
Presumed advanced intelligence: If alien civilizations are capable of traversing vast cosmic distances, it is often assumed that their intelligence and problem-solving abilities would be significantly more advanced than ours. It is expected that they would possess the necessary skills to navigate safely and avoid accidents.
Lack of evidence: Despite numerous claims of UFO sightings and alleged crashes, no conclusive scientific evidence has been presented to confirm the presence of extraterrestrial life visiting Earth. The absence of verifiable evidence leads many scientists to remain skeptical about the frequency of alien crashes.
It's important to note that these are general reasons and individual opinions may vary. If Michael Garrett has expressed his views on the topic, it would be best to refer to his specific statements or writings to understand his perspective more accurately.