Do you understand that this is a choice made by someone instead of a requirement?
You still need a metric shit ton of classes, so finding any one class gets harder. The chance of merge conflicts increases because of more lines per file.
You don't need to preface a method with `private` to be so but people still do it, why? My theory is the influx of Java dev influences, same with the "One class per file nonsense".
When working in teams, you really shouldn't put more than one class in one file, except for rare exceptions of very small and inconsequential classes. Otherwise the files get pretty big and the chance of merge conflicts get bigger.
The underlying problem - in my opinion - is that C# encourages you to overcomplicate things and that it is relatively verbose. Of course you can put a class with multiple members and methods in one line. But then it's also unreadable. Readability matters.
All languages with a syntax derived from C have a fundamental readability/verbosity problem, C# is no exception. F# for example doesn't have that baggage and is a lot more concise.