Developers who tried to build at least one product for an end-user know how many things are essential in a product: thoughtful UX, friendly UI, good performance and stability, security and data consistency, logging and maintenance, etc.
Multiply this to the number of platforms that you have to support. Add marketings and licensing, client support and bug reports, new feature requests, and competitive product pressure.
It’s hard to track everything in one head, and it is even harder to be perfect at every job. That’s why we work in teams. That’s why we use project management processes.