I can understand your point…
…but picture this: you personally have just discovered the existence of another spacefaring species. You know nothing, absolutely nothing about them other than the fact that they are a species that is able to cross the vastness of space.
The fact that they have interstellar spaceflight almost certainly demands that the species has the ability - and, more importantly, the desire - to spread. You do not know and cannot know if that other species gives any consideration at all for the existence of other intelligent species. You do not know whether they value peace and trade. You do not know whether they see humanity as an existential threat. You do not know whether that other species is able to exterminate humanity.
All you know is that the other species exists, and that they have the desire to spread.
Now consider this: once the other species discovers humanity, they will have the exact same set of facts that are described above. They might not see humanity as an existential threat to them…but they might. If they see humanity as an existential threat to them, what do you think they will do? Please note that you do not and cannot know what they might think, and what they might decide to do. All you know is that they’re an alien species that may or may not see humanity as an existential threat, and that they may or may not decide to exterminate humanity.
Sure, you could try to open up a channel of communication, but you know that by doing so, you expose humanity to an attack. On the other hand, you could attack them right away and gain the priceless wartime advantage of surprise.
Okay? You are personally in charge of making the decision. One choice is to reach out and offer peace and trade…and hope that the other species gives importance to the same. With this choice, however, you give up the advantage of surprise and open up humanity to the possibility of a war of extinction.
The other choice is to attack right away, to forestall any possibility that the other species might exterminate humanity.
Please note that the other species faces the exact same choice concerning humanity. How would they decide? You can’t know. And it’s the fact that you can’t know what they would decide, and that they can’t know what humanity would decide, that’s the real killer here.
How, then, would you decide?