Because if investors invest, it must yield in the form of interest, therefore, to make the value of the money that will be received in the future is equal to the money in the hands of the current investors.
If the investor to invest. It would have to return in the form of interest to make it worth the money to get the future value of money in the hands of the investor.
Because if investors put that money into the capital. Have to return in the form of interest so as to make the value of the money will be in the future value of money in the hands of investors at present.