My understanding is that the reason the number 30% is so magical is a historical anomaly. When software was physically distributed back in the day, 15% of the MSRP was reserved for the distributor and another 15% for the retailer. When these digital marketplaces were set up, the companies just said "well, we're the distributor and the retailer, so we'll keep both". Forgetting the fact that the cost to distribute and retail the software is literally pennies on the dollar of what it used to be.
I think the irony in this case is that this is a greed problem of their own making. When Steve Jobs announced that apps on the original iPhone would only be $1-$3, he set off the first enshittification crisis in the software industry. In 2008, Bejeweled cost $19.99 if you wanted to buy it on the PC. On the iPhone it was $0.99! This artificially low anchor price is what kicked off the adoption of ad and subscription driven software models in the first place.