Borderlands is not pulling its weight, there is no question about that.
However, BTA sales also drive up the average, though their strength is generally much lower than the top tier’s.
If the average is this low, it’s also telling us that the ratio between PWYW and BTA sales is not overwhelmingly dominated by the latter, and this is happening in the context of a low average which should be boosting BTA sales considerably.
Pillars and Tyranny have now been out for a while, so the audience for this bundle should mostly be comprised of fence sitters.
Four bucks should be low enough to get a good bunch of them off their fence.
Unfortunately, a not-inconsiderable portion of said sitters have likely had a chance to try the games, either because they got them as Monthly subscribers, or as freebies.
Since there is no DLC, the only reason for them to consider BTA is Cat Quest.
There’s likely other aspects worth considering; for instance, Borderlands, as a Diablolike-like co-op FPS, is a very different beast from wordy, D&D inspired, single-player RPGs. So those who are more like FPS die-hards might see little value in the rest of the bundle, and vice versa.