You say this now, but I've had to work on 5000 line (literally, 5k lines) classes in legacy code where there are a huge number of dependencies injected manually because the class is doing initialization for a huge number of components. You could argue that this was bad design but I'm sure it didn't start out like that, but just accumulated cruft over time and was too dangerous to refactor. (if prod went down it was at least a few hundred $k down the drain). DI makes it much simpler to wire things like this up without making mistakes like accidentally initializing the wrong class implementation, etc. In polymorphism heavy situations it reduces cognitive complexity a lot.