It’s cheaper to manufacture and maintain a single version of a product. It’s cheaper to ship and store a single version of a product. It’s also easier to adapt to quickly changing market needs if you don’t need to spend six months spinning up a production line for a different version of a product.
Also, the existing market for low-range EVs might not be large enough to justify the expense of maintaining a separate line.
If there is competition in the space, it’s safe to assume that some portion of these savings are passed on to the customer to better edge out competitors over price.
If you want to be very charitable: wealthier people who can afford the full-range version are partially subsidizing the lower range (tighter margin) version for more budget-conscious consumers.
Edit: Especially when talking about the structural battery of the Model Y, it may help to understand how these packs are made: www.youtube.com/watch?v=ozesI3OZEG0
The batteries themselves maintain the rigidity of the pack. If they removed some, they’d have to slide some dead weight in there. Also, once the packs are sealed, it’s impossible to remove a portion of the batteries without destroying the pack. These are designed features developed to reduce the overall weight and cost of manufacturing the pack.