Why am I replacing cv boots and axels so often. This will be my 3rd time for each side. I have been using after market parts.

The parts usage can sometimes be a causal factor. the close proximity of the catalyst can be as well. The independent subaru shop near me uses a mixture of parts to correct issues they find on the cars they repair. Their owner and their techs are all previous subaru people. They use factory make parts, but they have a unique perspective and are not bound to use the ones that keep failing. They have update info and procedures in place to keep subaru owners on the road. You could find the same type shop in your area perhaps.

I will need to repair this for the 8th time I my 2007 Tribeca. Why is it happening so often?

Referr to the first answer or show it to your subaru specialist.