re: Is because they don't exist or/and because we can't recognize them
Other reasons could be because they could now be extinct, or are with us but are unable to contact us, or have not reached up to our intellegent potentical.
I think if they are with us and have the means, then they are capable, unless they don't want to reach out OR, like us are unable to. With radio astronomy, it would take a while, in the visual, if they've seen proof of us being here, it would take a while for them too.
