Most previous Ryzen 3000 models received some sort of WRAITH cooler, however, the upcoming Ryzen 7 3800XT and Ryzen 9 3900XT will not get boxed with a cooler.

At least if the latest leak is correct. It could be to shave off cost, and well if you purchase a more enthusiast cooler, you're likely not using the included stock cooler anyway. Also, it simply can be TDP related. AMD has done something similar with the 16-core part, the 3950X. This all is based on a spec sheet that momomo_us spotted, Product codes for the two SKUs are marked as "100-100000277WOF" for the 3900XT and "100-100000279WOF" for the 3800XT, with "WOF" indicates "without fan" and thus no included bundled cooler.

As shown in another leak, the Ryzen 5 3600XT would include a Wraith Spire cooling solution. Also, it seems that AMD is pricing the 3900XT at 588€, the 3800XT at 463€, and the 3600XT at 287€ 

AMD Ryzen 7 3800XT and Ryzen 9 3900XT Will Not Get Coolers Included