Answer
It's 32-bit for historical reasons: when IEE was first built, 32-bit operating systems were still common, and various IE/COM-dependent integrations were significantly harder to support reliably at 64-bit — the second reason mattered more than the first, and remains a practical constraint. On Apps10, IEE has /LARGEADDRESSAWARE enabled, which lets it use the full 4GB of address space a 32-bit process can theoretically address (rather than the 2GB Windows would otherwise reserve half of by default) when running on a 64-bit OS — but it can never use more than roughly 4GB total, regardless of how much physical RAM the machine actually has (8GB, 32GB, or more). That extra RAM isn't wasted, though — it remains available to whatever else is running concurrently on the same machine (other applications, OS tasks), just not usable by IEE itself.