Why is there no universal methodology for running an RFQ/RFP?

We work with over 100 clients who use our eSourcing software to run their strategic tenders. Yet, almost every client has a slightly different process for doing this. For example, some will get an NDA upfront, some do not. Some will take an iterative approach, RFI > RFP > RFQ with suppliers declined as they progress through the stages. Some will send the whole tender documentation out at the start of the process. I have 2 questions; Why isn't there a standardised process for running strasategic tenders across the board? What do you think is the ideal tender process?