"As a toxic mechanism, cry proteins bind to specific receptors on the membranes of mid-gut (epithelial) cells of the targeted pests, resulting in their rupture. Other organisms (including humans, other animals and non-targeted insects) that lack the appropriate receptors in their gut cannot be affected by the cry protein, and therefore are not affected by Bt."
https://en.wikipedia.org/wiki/Bacillus_thuringiensis
Edit: "Spores and crystalline insecticidal proteins produced by B. thuringiensis have been used to control insect pests since the 1920s and are often applied as liquid sprays and donut pellets."
The end product is produced in large submersion fermentation tanks where the bacteria is first grown, then then forced to sporulate and allowed to ferment which produces the Cry/Cyt toxin crystals within the spores. Going through these phases requires precise control over nutrients, pH, agitation, and electron acceptors. Even activating the spores to create the inoculum and begin the reproductive cycle is nontrivial because of how sterile everything needs to be.
I did skim read a few papers on the subject and honestly I'm wondering how this stuff even grows naturally, everything has to be so specific it just doesn't seem possible at all.
My non-SME answer to this thread is:
1) 100 years is not that long in evolutionary terms; 2) cropland is large but not all land; 3) evolution is complex; 4) resistance is actually already observed in some species.
Evolution is complex and 100 years is not very long. But eventually some effect is likely.