You could - if you have several hundred full time developers working on it for years. This probably means you pay them, but the count of full time developers is the important part.
That cant be true. Why does QT need hundred of developers when the core product can be done by a single person? (see Sciter)
Details are hard. The core might be doable in a basic form by one person, but there is a lot more than the core that needs to be done. All those little details add up - they are only a small improvement, but all those small improvements adds to polish.