Most likely it was a deliberate technical limitation. After all, dialog windows themselves were already overlapped. I remember well what a headache it was to program and render graphical elements on those old machines (PC AT 80286 with 256 KB of RAM).
Despite the constant screeching for donations, the entire site is owned by a company with shareholders. All the “donations” go to them. They already met their funding needs for the next century a long time ago, this is all profit.