The problem is you are a small provider, and they are a megacorp. There are alternatives for them (all the other small companies), and the corporate terms are say 90 days. No small provider has the leverage to say no. The terms also just aren't up for negotiation, these are our payment terms.
The best I manage is having projects wherey ou bill half up front, and half on completion (or similar), though some companies also don't allow that.
The onyl time that changes is when terms ae totally ridiculous like GE and the 120 days. At that point it just isn't worth it. One client has annoyed me so much with late payment etc. forms, and just being a pain, that I've refused to deal with them again. It isn't worth the time.