Why interest repayment is always at the last month of the cashflow?

The interest payment comes in at the end of the cashflow as more of a technicality. It doesn't affect the amount of interest, as it stops being calculated when the loan is repaid.

It goes in at the end of the cashflow to avoid any confusion if the loan gets drawn down, repaid and then drawn down again. The main point here is that it doesn't affect the calculated interest amount or profit figures.