I hear your point, but this is more around Apple's "premium" product and pricing strategy. Obviously we as the consumers can ask for whatever we want (and getting more for less is always better for us), but realistically, I don't think Apple wants (or needs) to price their laptops so competitively (and "offer 16/512 in a $1300 laptop" as you put it)....but in 2021, 16GB RAM/512GB isn't too much to ask for in a $1300 laptop. Look at all the people in this forum asking whether they need 8GB or 16GB or how much storage they should get... at current RAM/storage prices that just shouldn't be an issue unless you're buying cheap - and even Apple's "entry level" systems can't reasonably be called cheap.
And as @Abazigal said, given there are plenty of users out there where 8/256gb is enough, part of it is intentional to not "overserve" the market and force those who don't need 16/512 to "overpay" for something they don't.
I also admittedly don't know Dell/Lenovo/HP offerings and whether their "premium" laptop lines include 16/512 as the base option in 2021. But my gut would say that they also don't offer that in the base option either, although their upgrade prices may be more reasonable than Apple's.