I don't know what to say - I've had several show up across both default maps, and it looks like others have as well.
Recall that there are several different crop-types, of which grass is but one - the one needed to trigger the mowing/baling missions. If we assume that each crop is weighted equally, then there is a 1-in-12 chance that grass will be selected for any given field. That's less than 10% chance (8.333%) for any given field, noting that there is only one chance per growth cycle.
IIRC, I seem to recall that root crops were negatively biased in FS17, so as to be less likely. The same could apply to grass, lest some players end up gazing upon a map full of nothing but grass!
From a gameplay perspective, baling missions are some of the more intensive ones, especially hay, where you need to:
- Turn over (using a tedder)
- Windrow (optional, but highly recommended)
- Transport (can be multiple runs, potentially across the entirety of the map)
So this could be the basis for such a negative bias as well.
Haven't seen a straw baling mission (or as a component to a harvesting mission). So am unable to speak to those.
edit - removed logical inconsistency