Because people have longer memory than you would like them to have.

What you promised 4 years ago constitutes a verbal contract. You did not say in that promise that the conditions would change. You promised delivery with less than a year timescale. On such timescales, compund or non-compound interest does not make that much difference. But four years is a long time. A 2% compound monthly interest over 4 years results in 259% of your original investment.