First off, Attack Attack and Asking Alexandria are about as far as you can get from metalcore while still being in a "Hardcore" genre. Killswitch Engage, As I Lay Dying, Children of Bodom, Every Time I Die, or Devildriver are better examples of "actual" metalcore.
Asking why it sounds the same is a relatively stupid question, however. It's like asking why does all rap sound the same, or pop, or rock. They share similar elements, and to notice the differences you need to dig deeper into the subtleties of the instruments, the chords, the timing, the pitch, and the content. For instance, many people say that Miss May I sounds exactly like the newer things by As I Lay Dying, and while I can see the similarities, they both have rather noticeable differences in the way they perform. I will try to give examples:
Killswitch Engage: They're the "mainstream" of metalcore. They are the generic sound that most of the genre typically strives to emulate.
As I Lay Dying: Not too far from Killswitch, but they are a bit more visceral in their performance and the melodies work much better as contrast to me.
Children of Bodom: They're set pretty far from the mainstream metalcore, because they stress guitar playing more than anything else which tends to usually be covered up by a mentality of "We need to make this sound as heavy as we can" in other bands. CoB understands that talent is something that shouldn't be squandered just to be "popular".
Every Time I Die: They've changed a lot over the years, their original stuff was closer to "metalcore" and now they're more "pop-metal" like A Day To Remember, but they still are not anything close to the sound of Killswitch or Bring Me The Horizon. They (In their older works) tended to go for an all-out sonic blast of sound (The "We need to sound heavy" mentality) and have definitely incredibly refined their sound in the last few years.
I could keep going on and on and on, but with what I have already said, I'll put a TL;DR
If you don't actually acquire an ear for picking out specific instruments and all of the little nuances that are different between each band, then everything will sound the same to you.
But please, don't base the entire "metalcore" genre off of Attack Attack!, Asking Alexandria, and Bring Me The Horizon, only one of which is even arguably in said genre.
EDIT: And while I don't agree with CAW on all metalcore being terrible, I will grant that there are simply way too many sub-genres now and it's become like attempting to navigate a labyrinth of entirely mislabeled items in the worlds largest library. In fact, I think it's the exact opposite of what has happened with the gaming industry: The Adventure label is no longer used anymore so Adventure games get pigeon-holed into the wrong genres. In metal, there are so many sub-genres that bands are struggling to even know what genre they are currently residing in.
Thinking about it, that could also mean the same thing is happening in both, just one is shrinking and the other is over-inflating...now it's getting convoluted, so I'll just stop.
EDIT EDIT: If you want a band that technically falls into the metalcore genre but definitely mixes things up, check out The Human Abstract. The main contributor to the actual songwriting (and lead guitarist, I believe) for the first and third albums by The Human Abstract went to a university to exclusively study classical music. As in, Tchaikovsky, Bach, and the like. So. . .yeah. They're not like Bring Me The Horizon.
What hardcore are you listening to?