Why are there no easy-to-solder SMD diodes in 0603 packages?
0603 is 1.5 x 0.85 mm.

There are diodes available in "SOD-323" (aka SC-76) package, which is 1.7 x 1.25 mm, pretty close to an 0603. Digikey lists ~250 part numbers, from over a dozen manufacturers, for single rectifier diodes in this package size.