One theory* used to explain why we have found no definitive evidence of intelligent extraterrestrial life (despite the probability of it existing) is known as the Zoo Hypothesis.
Wikipedia: “The hypothesis is that alien life intentionally avoids communication with Earth; one of its main interpretations is that it does so to allow for natural evolution and sociocultural development, avoiding interplanetary contamination, similarly to people observing animals at a zoo. The hypothesis seeks to explain the apparent absence of extraterrestrial life despite its generally accepted plausibility and hence the reasonable expectation of its existence. A variant on the zoo hypothesis suggested by the former MIT Haystack Observatory scientist John Allen Ball is the laboratory hypothesis, where humanity is being subjected to experiments, with the Earth being a giant laboratory.“
You can also watch a short (3 min) video of the evolutionary biologist Bill Hamilton discussing the topic (back in 1992) here: https://youtu.be/IjnvNT5ARTk
*Other reasons that we have failed to make contact with off-world civilisations include:
- There simply are none.
- The distances between us are too large and no civilisation has technology to traverse them.
- They are not interested in making contact.
- Most civilisations end up destroying themselves before becoming advanced enough for interstellar travel/communications.
- Extraterrestrial life is so advanced or so anomalous to our current understanding of the universe that we simply do not recognise them.