At a $188 price point. An additional 4GB of memory would probably add ~$10 to the cost, which is over a 5% increase. However, that is not the only component they cheaped out on. The linked unit also only has 64GB of storage, which they should probably increase to have a usable system ...
And soon you find that you just reinvented a mid-market device instead of the low-market device you were trying to sell.
4GB of ram is still plenty to have a functioning computer. It will not be as capable of a more powerful computer, but that comes with the territory of buying the low cost version of a product.