yup, time...how long have we been around, 300,000 years?
A drip in the ocean to how old the universe is and will survive until all the lights go out.
And you can never confirm the current existence of life.
Suppose a radio telescope on earth receives a signal that says, “Hey, we have high hopes for your planet that just formed! Get back to us ASAP!”
That message was sent perhaps three billion years ago from a planet three billion light years away and we respond, “Guess what? Donald Trump was elected president!” They get that message, if they still exist in three billion years, and by the time they get back to us in six billion years. we will not exist.
It would be like a very slow AOL Chat Room.
Perhaps more to the point; for how many of those years have humans had the ability to count past ten?
You don't have to go back very far, if at all, to find ridiculous things that humans believed.
If humanity lasts another 1000 years, we may then have a very different picture of who we are, where we came from, and where we are going.
The Drake Equation perhaps needs some terms to reflect the limited capabilities of those who are using the equation.