The fact that it allows them to put the pipes needed for all those items in the same place probably also helps. In a lot of places near me, the kitchen sink shares a wall with the bathroom because it's cheaper to do the plumbing that way. If the bathroom and kitchen were on opposite sides of the place, that means more work plumbing wise.

Plus, walls take up a lot of space. If we put the toilet in a different room, that would mean losing more space to walls (and configuration options). In my bathroom, the sink is in the front followed by the toilet followed by the shower. Walls (and the clearance that the doors would need) would mandate using a lot more space. Plus, you'd need more space for each bit not to feel claustrophobic. You don't feel like you're in a tiny box at the sink in my bathroom because it's open to the toilet and shower. If each section was only 3ft wide, it would feel really claustrophobic.