That link was pretty irrelevant to your original question.
The answer is that even cash-settled trades take 2 days to settle, which most traders would consider an unacceptable wait to take delivery of their shares in this day and age. Seeing the shares appear in your account immediately after buying is merely an abstraction provided by brokers, and the brokers need to have their own cash on deposit to secure this.
Strictly speaking, most traders of US listed stocks wait two days to settle trades. Robinhood will give you an advance on your funds but that doesn't make T+2 settlement "unacceptable...in this day and age" lol.
The answer is that even cash-settled trades take 2 days to settle, which most traders would consider an unacceptable wait to take delivery of their shares in this day and age. Seeing the shares appear in your account immediately after buying is merely an abstraction provided by brokers, and the brokers need to have their own cash on deposit to secure this.