Is there any particular reason why you want 64-bit Raspbian? Or were you just curious about its absence?
I use Raspbian because (from what I have seen) it is far better suited to the Pi, better supported, has a recent kernel, etc than the various 64-bit versions of Linux available. It really is a good OS. For 64-bit ARM (learning AArch64 for example), I use other SBC's where the "official OS" is 64-bit on the same processor.
People expect 64-bits perhaps because its advertised as a 64-bit CPU
https://www.raspberrypi.org/learning/ha ... pberry-pi/, but really there is only 1GB of ram so its not needed for memory addressing. Otherwise its just about performance. Obviously anything involving 64-bit arithmetic would run much faster (such as sysbench!) and the generally more modern, streamlined, ISA with double the number of registers (32) helps most software a bit. The latest IEEE compliant version of NEON SIMD it includes, is available in 32-bit mode as well.