Maybe they're here, just waiting for us to catch up

One reason aliens are not contacting humans could be that we are not actually that advanced. And contacting us would intrinsecally give us sudden acknowledge of some level of technology or theories not even imagined till now.

Some of that tech could easily create a global imbalance in society leading the civilization to a collapse eventually.

The tech could already be around us, just evolving slowly being adopted by the global society, just as slow as most of the population needs to be it, to not collapse in a sudden explosion of changes subverting or erasing rules and status quo built through centuries. As we know from previous collapses, the rule of law and the need of societal stability to develop advanced tecnology is not easily achieved, it could take another 500-1000 years to recover from a neo-collapse ala Roman empire or Han dinasty.

Scify gives plenty of examples of revolutionary tech that if becames suddenly broadly adopted, it could tear apart most of the global societies structures.

It could be nothing like interstellar travel or wormholes, nor time travel, or else. I could be something more subtle, not inmediately possible to be perceived by homo-sapiens as a "danger Will Robinson" circumstance.

It could start simply like an - AI powered - app giving us 90% of certainty about if someone is lying in an audio sent in Whatsapp. Then you could start reviewing all recorded messages from politians since the 1930's to check if they were telling the truth or just plainly lying to their populations.

And then we could start just double-checking for lies everything said aloud everywhere.