I had the same thought but I also wonder if these highly trained illustrators were happy with making corporate renderings or if they had imagined themselves working in a more creative capacity?
I also don't think its gone. We still have great illustrators but someone somewhere has to decide to use illustrations instead of a photo, CGI, or something else and then they have to pay the premium for that service.
I know nothing about the industry let alone in the era, but I imagine 'drawing glassware for Pyrex catalogue' wasn't a full-time job - but either a temporary contract, or just one project for an agency. So you might view it as an opportunity to perfect your drawing of glass, or just a boring gig but paying the bills.
If you reflect on your own profession & career though... Well, rather than speak for you, I too 'had imagined [myself] working in a more [x] capacity'!
Who says they're not also being more creative elsewhere?
Plus wouldn't it be a sense of creative pride knowing that you can create an illustration that perfectly depicts refraction through glass, such that people find it hard to differentiate it from a photo? (which did exist in 1938)
To you second paragraph, the output of a CAD model is often used for line art of a product, and sometimes for an illustrated parts breakdown.
I also don't think its gone. We still have great illustrators but someone somewhere has to decide to use illustrations instead of a photo, CGI, or something else and then they have to pay the premium for that service.