Why is it impossible to use an individual homozygous for a dominant trait when analyzing crosses?

A recessive is used in analyzing crosses because it is not possible to determine a heterozygous individual using the dominant allele.
For example:
R-Aah AA R-Aa x aa
G – A, and A G – A, and a
F1 – AA, Aa F1 – Aa, aa
As can be seen from these schemes, in the second case, splitting occurs, and 2 signs Aa and aa appear. And in the first case, it is outwardly incomprehensible, maybe it is AA or Aa.