It’s really not. Other companies with socketed RAM also upsell, they are just limited in how much they can ask because the customer has the option to DIY adding more RAM. So the cost these companies charge is roughly the price to the customer of upgrading their own RAM, plus a bit extra for the convenience of not having to do that.

For example, Framework upcharges by something like 20-50% for RAM and SSDs when comparing to equivalent parts. It’s not just Apple, all OEMs do it, but Apple can charge much more because the user can’t easily replace either on their own.