If I was buying a PC (certainly for a desktop, possibly for laptop) I'd probably avoid Dell et. al. and go to a smaller supplier that really did build to order and where the upgrade prices were basically the retail prices for the components. So, one site is giving me the retail price of Samsung 980 Pro M.2. sticks as:
500GB: £80, 1TB: £100, 2TB: £264
...so it's pretty obvious (a) that the 1TB one offers the best value and (b) why it's not worth Samsung's while to make a 200GB option that would cost nearly as much as the 500GB one.
Laptops makers like Dell and Apple will be making similar judgements - with different numbers. As you said, laprtop prices aren't that tightly liked to the cost of individual parts, so the difference between (say) 1TB and 500GB at the sort of volume prices that Apple or Dell can command wouldn't necessarily require a significant price increase. Or rather that would be the reasoning if they were trying to make the best product for the price range. Of course, in reality it's about keeping the base price low while pulling in money from hugely marked-up upgrades, and hoarding every penny saved on the bill-of-materials. Which is life - but if it is taken to extremes and starts to result in inferior products, disappointed customers and delivery delays, it's a problem, especially with the long-term reputation of a brand.
Well, as for (B) - first, Apple doesn't really have a tier for basic personal productivity, communications, web surfing, everything in the cloud etc. - or, they do, and it's called iOS. Apple have chosen not to play in the <<$1000 basic laptop/chromebook arena. Second, we're not necessarily talking about Apple's lowest tier entry-level computer here - currently the M1 MBA - for which 256GB SSD/8GB RAM might still be sensible & more in line with the rest of the industry. Even the M2 Air is being sold at a premium over that, and the 13" MBP at an even higher premium (mainly for better performance at high loads - so definitely not entry tier).
And for (A) - that's exactly what Apple have apparently failed to do with the new M2 machines - it's no longer cost effective to use 128GB modules (whatever the underlying cause, it probably boils down to cost in the end) so they've significantly downgraded the performance by only populating one channel/controller/whatever rather than increasing the spec.
In the case of the Studio it's not so clear - but there would probably be a speed increase by populating both slots and we're talking about a few tens of bucks difference in retail price between 512GB and 1TB, a fraction of that at Apple-volume pricing, and the entire range is aimed at people doing more demanding & storage hungry audio/video/graphics tasks. I can't imagine why anybody specifying a computer for "studio" use in 2022 would think twice about specifying at least 1TB of flash at current prices - especially with SSD storage where there are speed and wear advantages to having plenty of spare space.
