Using the C# language and the .NET Framework you are still leveraging tools made by others. You're just leveraging different tools.
Professional studios rely on all kinds of libraries. There is generally no need to rewrite everything yourself. In the 1980s you needed to write your own graphics drivers for every card, your own audio systems that wrote directly to the memory locations and IO ports of the different audio cards. Over time more middleware came out where you could hit a generic driver for many different audio cards, or VESA drivers for a bunch of video cards. These days we hit systems like OpenGL or DirectX for graphics, we use systems like WWise or FMod for audio. And that can mean relying on major engines like Unity or Unreal as well.
There is nothing inherently wrong with doing it all yourself. You could even write your game entirely in machine code if you felt the urge. But doing so is directly at odds with productivity.
You can take advantage of systems that would take thousands of hours, tens of thousands of hours, or even millions of hours for you to develop on your own. And that is exactly why most groups do it.