Share your insights or perspective on the financial matter presented in the input.

why is buying trading-stock from cash not regarded as an expense? 
Because the stock still has the same value as the money paid for it - you are just exchanging one asset for another (of course the stock value starts to change immediately, but for the accounting the fictional value is the buying price). For the accounting, it is similar to changing a 100$ bill in five 20$ bills - same value, still assets.