Working with an extrusion is one of the things that seems easy but the devil is in the details. Its a product where the final finish can make or break its acceptance. In some cases it can be as difficult or more than mirror polishing. As you tend to see the grain finish through the top coating on an extrusion the margin for error is much smaller. If the finish is not even or straight it will at minimum mean rework or possibly rejection. When you apply the grain you have to be careful not to over round any profile and go just the right depth with the grain. Too shallow and the top coat fills it in and it vanishes, but too deep and coating ends up thin in places and the grain is too pronounced. With the cost and headaches involved with top coating extrusions rework and rejection need to be avoided.
Then the best part is you need to finish all sides in many cases. With bur removal or even polishing you can get away with one side. Well with extrusions you now need to finish four sides and they all need to look the same. You can see how this just gets better as you go further into the process. There are machines that can do all four sides in one pass but they tend to be complicated and expensive. You also end up with four or more abrasives to manage and adjust. A good top and bottom setup tends to be more forgiving and just as fast in the end. It removes some of the complexity and eliminates the additional abrasives. It will mean passing the part more than once but a simple roller setup allows it to be done quickly or it can even be automated. There are ways to improve the process and cut down rework and rejection rates but it is still not a simple process.
So the next time you see a window or another item with an extrusion with a perfect finish think of what went into it. There are many steps and problems to be overcome on what seems a simple part.