Apple started working on 64-bit application support back when they were still on PowerPC, where they only had 64-bit for desktop processors, not laptops. Then the first round of Intel Macs had 32-bit Core Duo processors. Less than a year later the second round introduced 64-bit Core 2 Duo processors that were pretty much a drop-in replacement, and at the time it certainly didn't make sense for Apple to overhaul their EFI, kernel and drivers with a 64-bit transition only 9 months after switching from PPC to x86.

Support for 64-bit GUI applications on OS X didn't arrive until 2007, a year after Intel Macs started using 64-bit processors. Support for running a 64-bit kernel arrived in 2009, and only for machines that had 64-bit EFI firmware. In 2012, Apple dropped support for the machines that had 64-bit processors but used 32-bit EFI, so from that point on they were handling the 64/32-bit mix the same as other operating systems.