Nah 60 days is nothing in some circles. If you do government work it can be much longer. There are some contracts that are so huge that there are multiple layers of subcontractors, etc. and each has terms with the next that require them to get paid before they pay the next person in line, and it goes on and on.
Sure, it's not really that fair or pleasant to deal with.
But there is another game going on that makes it worthwhile - that is the 'easy money for business that aren't cashpoor' game that you see on huge projects. If you are one of the vendors and you constantly complain about your 30 day-term being violated, you are considered an unprofessional vendor who is desperate for cash. If you just wait, and wait, and wait until it's been 90 days before you even mention the payment, then you start to be friendly with the PMO office since they are struggling to manage cash just like everyone else.
And if you can keep 2 or 3 subcontractors on salary and make sure they get paid even if you don't, you are in a position to make some real cash.
Once again, cash is kind and a well capitalized business has so much advantage over one that is scraping by.