The primary reason is weight, If a gear drive motor were mounted out on the forearm of the robot this would decrease the J3 payload. In its current configuration with motor on the opposite side of the J3 axis the motor weight acts as a counterbalance, and the linear drive motor weighs less than a motor with planetary gear box. The other reason is width of the forearm, If a nema17 gear drive motor with motor mount were mounted sideways on the forearm the rear of the motor and encoder would protrude several cm out the side of the J4 forearm assembly. I also wanted to reduce the amount of wires that had to be run past the J4 rotating assembly.

I was thinking something similar to this:

I asked just because i believe that this way would be much more simple to build.

thank your sir for taking the time to explain it to me!

By building it this way you would have a belt in the way of the joint 4 rotation. It would reduce greatly the J4 range.