Because agile is a project management process, not an engineering practice. The value of sprints is in delivering product at the end of every sprint. If that's not happening, because the client isn't interested, and you're not getting product feedback from your customer who is the only person whose feedback actually matters, and using that feedback to determine the tasks that go into the next sprint (including potentially cancelling tasks for work the customer is no longer interested in), then you're actually slowing the project down by forcing people to work on fit and finish every sprint before they need to (i.e. project completion).

That's not to say that you shouldn't anyway have good engineering practice, like short-lived branches and continuous integration. But you should be merging in branches on a schedule that is independent of sprints (and hopefully faster than the sprint length).