Why are the currents not adding up to the current going through the R1 resistor? Why is it the same current in loop 1 going through the R1 resistor? What is preventing the current in loop 2 from adding up with the current in loop 1 in the the R1 resistor?
The current from current source I1 wants to get from positive end of current source to the negative end as quickly as possible with the smallest effort possible. So, current travels to R2, then goes to that node/junction and decides: Where I am going now? To the right, where I see my destination directly, or down to R1? Why should I go to R1? I am lazy, I want to go to my destination directly. And so current flows from that node to negative end of current source.
There is nothing that would attract current to go through R1.