Dialga and Palkia weren't summoned. They were forcibly pulled out of their dimensions. If you looked where they were standing after you wake up from being knocked out, you can see the tear in space into their dimension despite the fact that they left. The whole purpose of summoning Palkia and Dialga was to distort the balance and cause the current reality to collapse in order to make a new one using their power. You really think that didn't weaken anything in the fabric of space-time?

I think you completely missed the point I was making. If the fabric of reality is weakened, then whatever was sealing Giratina in its world was also weakening allowing it to punch a hole through and get to the real world.

But of course, completely disregard what I said if you want to make your point, even though the fact remains that Giratina only appeared in two places, one is where there was already an open portal and the other where reality was being distorted.

Also notice that when Giratina appeared from his make-shift portal in Spear Pillar, it appeared as a shadow and not in its full glory. All it could do was appear as a shadow and drag Cyrus into its world, it couldn't fully materialize. As for Giratina's motive? Its a renegade Pokemon, by punching that hole, it allowed the Distortion World to slowly spread. That's the whole reason Cynthia and the protagonist had to go through the portal, so that they could fix it. Now if the Distortion spreads. Giratina would have full reign over the normal world as well.

Nothing you said also changes the fact that Giratina did not in fact walk in and out of the Distortion World as it pleased