But why wouldn't they want to "interfere" with our developement?
Sure, many people would freak out, but if they have the instruments and are benevolent they could help us enormously.
Given that they already made contact with us, and that they don't officially show them self, i don't think this makes a possible answer.
The only circumstances in which this could be the case is if they want to study our evolution through the centuries , or the they simply want to enjoy the shit show
Fantasizing a bit, i came up with these other 2 options:
1) their contact could represent a Phisical/Mental/Spiritual threat to us (ex. the bacteria they could bring)
2) their way of communicating makes it impossible for them to start a dialogue with us.(For example, they could exchange information through biochemichals, or psychic powers).