Because Developers are not the only stakeholders. Management and marketing needs to know when the next release will be. In a complex project it doesn't matter if you are ready to go with your part if someone else just merged in a new feature that is crash prone in situations they didn't think to test. In a complex project you may also depend on hardware being delivered at the same time. In a complex project there may be people planning when they can safely role this out to their system.
Agile works great for a small project where a few developers is all you need. However few in the agile manifesto seem to have thought about how to scale it to larger projects. That isn't to the manifesto is wrong - it very much fixes some real issues with previous project management. However some of those real issues were an intelligent response (not to be confused with good!) to even more complex issues that you cannot ignore just because they are causing issues.
Scrum is one of the few that thought about the large project problem, and so it appears at first you can scale it to larger projects. However now that we have tried it for a while I can safely conclude it isn't a great fix. I'm not even convinced Scrum is better than waterfall (waterfall has iterations - version 1.0, 2.0...), though scrum and waterfall get different things wrong with project management.
Of course the next question you should be asking is how do you manage a large project. I do not have an answer. I haven't even seen evidence that anyone has found a good answer despite many smart people facing the problem and thus thinking about it.