The price of milk is not generally subject to speculative bubbles, though. When it is (let's say some kind of shortage rumor or disaster prep) the price indeed crashes after reaching absurd levels.
I wouldn't call where we are a speculative bubble. If you look at the P/E or dividend rates of stocks and compare that to other investments you'll see they are still favourable. So a decision to e.g. own MSFT stock that a higher dividend than you'd get in a 10 year bond, while not without risk, isn't pure speculation. MSFT has a business with good prospects and it makes money and pays it back to you. Where else do you put your money?