This is the crux of the issue for writers: what used to be single, extended year-long gig in the age of broadcast TV has become a series of short-term gigs lasting a few months.

Each show writing gig lasts about 2-3 months for streaming, 3-4 months for network half-season, or 5-6 months for network full-season. (These timelines reflect the duration the writer is employed; the show's full pre-production and post-production timeframes are usually a lot longer than this.)

Also, good people in this field aren’t sporadically employed and keep a pretty good book of business.

This is definitely false. Variety and The Hollywood Reporter have interviewed a number of writers as part of this strike. A number of former showrunners of successful broadcast shows and film blockbusters have been affected by this. It's not an issue of "book of business" like it is with other industries; the problem is that the work itself is simply much shorter in duration than it was in the past.

For comparison: it would be as if your coding projects that used to last for a few months now only lasted for a few days, but you got paid the same rate by unit of time as you did before (i.e., you now only get paid $5k for what once would have earned $100k).