You only need 10 million years to colonise a galaxy and we may well do that.Why would we have seen them? You expect ‘them’ to colonise the whole galaxy when we can’t and probably never will
We’re absolutely brand new and there’s billions of years before us. The fact someone hasn’t done it means either life doesn’t exist or intelligent life doesn’t elsewhere.
Unless we’re in what’s called the Zoo Hypothesis, which means they’re out there and watching us but there’s a rule that they leave us be.
That said, you’d expect a million year old civilisation to be visible right across the night sky, Brian Cox said it’s a mystery there isn’t one.