Hi @Hamada Ahmed
The technical reason is that ACI is a CLOS fabric which is a mathematical design intended to facilitate non blocking communication.
If you think of your ACI fabric as a big modular switch with a backplane (the spines) and multiple line cards (the leafs), its the same reason you don't (generally) connect a line card to a line card.
Here is the original paper by Charles Clos in 1952.
https://archive.org/details/bstj32-2-406
.. and a great summary from this Nanog presentation
Demystifying DataCenter CLOS networks
https://www.youtube.com/watch?v=GVT3KeAa9xw
