A specific other person willing to pay you a certain amount of money to buy something from you, it does not necessarily mean that "thing" has increased value because there maybe not a majority of members in the society recognize that amount to be reasonable.
Take the 1963 Ferrari 250 GTO which was sold for $70M in 2018 as example, assume you own that car and you want cash, is it guaranteed that you bring the car to the bank, and they will be OK to take your car and give $70M to you? I highly doubt that. So that claimed $70M is not 100% guaranteed. How come you consider something not guaranteed and even may not work to be its real value?
We can look from another perspective: suppose I buy a $1 pencil from Walmart, and I sell it to my son for $1,000, and then my son sell it back to me for $1,000. Then we have 2 transactions for this pencil with deal price of $1,000. Do you think the pencil's value has appreciated to $1,000?
Same reason for cars. Car collection is highly illiquid and individual specific/restricted (relatively high cost and special circumstance to match mutually agreed buyer), which indicates high liquidity risk and its real value is always discounted by the corresponding risk factor, and the risk factor fluctuates wildly because it is not backed by any credible government entity.
Gold, cash, (mostly) real estate value have real meaning and stable because they are publicly accepted by everyone in the world and backed by the government. Especially for US dollar, due the US government's power, it has extremely low liquidity risk, so discount factor = 1, which means real value = face value.