I don’t know why manufacturers charge so much for spec bumps (this isn’t limited to Apple). Is it really only because they can?
This also played into sale or discount strategies: if the same model becomes cheaper over time but list pricing doesn’t adjust, you’re making a good bit more from everyone who isn’t price sensitive even if you allow resellers to discount it periodically or offer better pricing to business orders (“free RAM upgrade”).
This is addressed in TFA, which suggests it's a side effect of smaller 128GB modules becoming scarce (or maybe more accurately, "not reliably available at Apple volumes").
The article asks "why is Apple even bothering with a 512GB version?", and the two answers are: (1) To create a cheap-ish "price anchor" config, and (2) to serve people for whom price is paramount, who are most likely doing tasks like browsing the web, doing email, and using office-type apps.
While their absolutely is benefits to how they handle their RAM the upgrade prices are nuts. $200 extra for 8GB more, 64GB requires a CPU upgrade and costs $400 extra over 32GB. I've heard the CAMM modules Dell developed and seem poised to replace SO-DIMMs in laptops will allow for faster speeds and replaceable LPDDR too with less Z height and eaiser board design so hopefully we will see an uptick in PC laptops with replaceable memory again.