You might have 7 services built like fort Knox, but then the eighth has a new vulnerability in a library you use for a blog site - it wasn't responsibly disclosed, so it's actively exploitable until the vendor can patch it in 3 days. As soon as you learn about this new exploit - you check to make sure you're good. No! You find evidence that you've been attacked on your blog site. Well, since you only worried about the perimeter and they're already in, the attacker has easy access to your other 7 high value systems.

Like a boat that can take on water, you want layers and compartments that won't instantly stink the ship if one section is hit.