Serious team wants a set budget and feature set. They also want a dialog with the aquiring party, because as you dig deeper in the software you uncover oddities. These oddities are more often than not a failure of the aquiring party understanding of their own business operations.

And thus, a serious team will help the aquiring party refine their business process by either removing useless steps, adding missings steps or changing a step in the overall workflow. And that's were the most of the value of making a new software comes from.

Doing waterfall will stop this from happening and will remove actual value from the software because it's going to be bloated with useless things that were badly understood by the aquiring party.

Agile is about producing as much value as possible, as fast as possible, in a set budget.

English is my third language so sorry if it's hard to understand or feel aggressive.