The simple answer is because these bearings handle both loads.
There are many different types of bearing construction. The most common is the ball bearing, as used to support the main shaft of the Pantera 50. These types of bearings are designed to accept 'both' radial and thrust loadings.
A bearing is sized by the engineer with the loads it will see in mind, plus a margin of safety. Hence, due to the size of the main shaft bearings we use, you get more than adequate handling of the thrust loads encounter in flight because these things fly around at paltry 8 pounds, give or take, and even with several g of acceleration it's simply a non-issue because this is well within the capability of even a single Pantera main shaft bearing - and we use three!
Now if the question is, would it be better still with a thrust bearing? To this I would offer a qualified - maybe. This is because we open another can of worms related to complexity and costs because the thrust bearings would need to be paired, i.e. one for upright and another for inverted loads. But this results in a more complex, product, which weighs more, and increases maintenance and doesn't deliver any increased benefit at the loads we see.
Moreover, while I've seen add-on thrust bearing kits, we view them as largely ineffective - in practice no more effective than what we achieve with ball bearings. Thus, I have refrained from offering them as an option.
Finally, bear in mind Audacity Models' engineering philosopy, which can be summed up as . . . the fewer parts the better. Consequently, any conceivable improvement in load handling via dedicated thrust bearings is overwhelmed by added complexity and weight, additional cost, and additional required maintenance - especially since they'd deliver no practical benefit whatsoever!
Hence, as usual the answer is, 'It depends'. I hope this helps and thanks for asking.
AMA # 47381