Because you don’t know what you need.Great news.
But in general, I don't quite understand the fuss around user-upgradability. Why do storage and memory need to be upgradable? Just buy as much as you need over the machine's lifetime right away. Think you will need 32 GB memory in the future instead of 16 GB? Then just get 32 GB memory now. Think you will eventually need 4 TB storage instead of 2 TB? Then just get 4 TB now.
A machine that is good enough at the time when you buy it will also be good enough five years later.
Eventually after some years, the CPU becomes the bottle neck, and since that is not really replaceable anyway, whenever that point comes, you will need an entirely new machine anyway, not just a memory or storage upgrade.
I’m still using a MacBook Pro from 2011! It was base spec at the time and suited me well. A few years after I bought added more ram and a ssd because it was too slow.
11 years later it’s slow but enough for what I need again as. The 2011 could not get an SSD or 16GB or ram. So I literally couldn’t buy what I needed later.