I recall something my father said once about how people tend to have blind spots about their enemies. The thing is, we all know that if our own country (whatever that may be) were attacked or conquered, it would just fire up our anger and national pride and inspire us to fight back all the harder. Yet somehow, countries throughout history have tended to assume that the same would not
be true for their enemies, because their enemies are inferior and craven and would thus be cowed into submission by a show of force. So countries keep attacking or oppressing their enemies in the belief that it will defeat and demoralize them, but it usually has the opposite effect and backfires upon the attackers or oppressors. So Imperial Japan bombs Pearl Harbor and as a result we dismantle the empire; and conversely, the CIA helps overthrow a populist Iranian leader in favor of a US-friendly dictator, and as a result there's a populist revolution that turns Iran into one of our biggest enemies. Not to mention all the dictatorial regimes that have oppressed and terrorized their own people in the belief that it would keep them docile, only to provoke them into revolution instead.
You'd think that eventually people would learn to expect this sort of thing, to know that attacking or oppressing another nation -- or their own people -- would just backfire and lead to their defeat or overthrow. Yet century after century, people still don't get the message. They still assume that the other guy will react differently to threat or oppression than they themselves would, that the same things that would inspire themselves to greater determination and rage would instead break their enemies' spirits and leave them defeated forever. It's a bizarre but perennial blind spot.
So maybe that's why the Xindi attacked Earth with the test probe rather than some uninhabited moon somewhere -- because they thought it would demoralize humanity and leave us too afraid to strike back. It doesn't make sense, but then, it never has before, yet people keep trying it anyway. It's just the blind spot of xenophobia, the assumption that your enemy is fundamentally different from yourself and doesn't have the same virtues such as courage or patriotism or love for family to inspire them to fight back.
Philosophy aside, there's also the fact that Earth would never have known where the attack came from, or where to find the Xindi, if Future Guy hadn't tipped off Archer. So as far as the Xindi knew, Earth should've been unable to identify or locate their attackers in time to prevent the final attack. So they would've deemed the risk of retaliation low enough to justify launching an attack for the purpose of demoralizing the enemy.