Why do people buy any investments knowing that someone bought it waaaaay cheaper than they did? Because they look at the future not the past. Thats what smart investors do. We still don't have all the smart investors on board yet due to various reasons... mainly the ability to acquire legally and large amounts. Once that wave is over, bitcoin will have grown and will be used for what it is, and people won't worry about it as an investment that someone bought waaaaay cheaper than they did a long time ago. They will think of it as an amazing technology. You got to remember bitcoin was not made to make you rich!
They don't if it's really an investment, which is basically defined as loaning somebody who does something productive money for interest. Out of it comes something of value, a product, a service, knowledge or information.
That doesn't happen with Bitcoin because the mtoney you used to buy Bitcoins goes to another person who doesn't use that money to earn something for you. That's why it's called speculating and that's why this board is called as such. We are speculators not investors.