Hi Mark,
I kinda suspect why ESRI does not allow to export layout as hi-res georeferenced raster.
There is a workaround though.
(1) Export layout to pdf with georeferencing information checked on. Make sure dpi is at least 300 and maximum detail. I am also setting pdf compression to something lossless - like LZW or none - to preserve details.
(2) Use PDF to TIFF command to extract geotiff
The only thing - Arcmap Desktop bombs out when exporting large layouts (paper sizes 36x48 inches and larger) with full detail and higher dpi - "Not enough memory to create metafile" since it is 32bit app with not very efficient memory management.
So another workaround is to use Arcgis Pro to exporf pdf. You still can run into limit again - "GPU Resources exceeded..."
Good luck