Why in the vertical calculation of decimal multiplication, the writing format of two multipliers is not the common digit alignment, but the last digit alignment?

Why in the vertical calculation of decimal multiplication, the writing format of two multipliers is not the common digit alignment, but the last digit alignment?

The basis of decimal multiplication algorithm is integer multiplication. The column vertical calculation of integer multiplication has a certain foundation for students, but how to make students understand that "the calculation method of decimal multiplication is the same as that of integer multiplication" actually has a very important link: how to make students transition from column vertical calculation of integer multiplication