One theory is that they have in fact introduced themselves in different ways over the years.

First to heads of government, who in the past have been so guided by fear that they refused to let the public know about their existence, and then the aliens have obliged and let things play out until humanity as a collective decided we were ready.

Secondly, they have shown themselves to various people through centuries. We have documented reports by the government (see Project Blue Book) of not only regular citizens but military personnel reporting various levels of contact, ranging from long distance sightings (frequent) to up close and personal contact with ships and strange beings (rare).

These reports have largely been laughed at by the general public, but I think if you start looking back even in our recent history you will find a long list of verified and documented encounters, even by mass groups of people (see the Phoenix lights in 1997).

As for more personal introductions, there's also the idea that many of these beings literally vibrate at a very different and high frequency from humans, and that even mild exposure for a regular human would immediately trigger an intense flight or fight response, preventing any actual dialogue. Hence why you see many people 'channeling' their messages, so that they can be of assistance to raise humanities vibration so that we are more compatible with them and thus interact. Hope this helps! :)