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It's a place where people live and work, so while it shouldn't look abandoned, it shouldn't look continuously pristine, either.

Sure but if there's still someone living in it then why wouldn't they keep it clean

There will have been different standards of "clean" when you don't have our modern tools for keeping things clean. The exterior areas, for the most part, will have been as clean as a broom can get them.

Particular pet peeve of mine are stables ostensibly in active use, yet with floors as clean as if they'd just been powerwashed.

have new furniture every now and then

Of course, if they're wealthy enough to have fancy furniture made they will want to display as much and keep their furniture in good shape. Still, there's a difference between handmade furniture looking like it has been used, but well-kept, and brand new furniture made with modern machines.

Also, I find it hard to believe that the guardhouse or servants' house or other buildings within or around the castle would maintain the same standard.

Either way comment wasn't aimed at the furniture and equipment so much as the general shape of the fortifications, which is a lot more labour-intensive to keep in pristine condition than getting a new set of chairs.

I still have clean curtains.

Handspun, -woven, -tailored and -washed?

We take some amenities for granted, but making cloth was a lot of work before the Spinning Jenny came and made the most arduous part (spinning thread) a lot quicker.

As above, wealthy nobles will have displayed that wealth through conspicuous amounts of fabric in their clothing, tapestries, curtains and more, all kept clean and in good repair. That can be used as a visual device: a noble whose house is bare is in dire straits.

Peasants on the other hand tended to wear things for as long as they could, then turned them into cleaning rags or whatever other purpose they could find. You wouldn't expect a white, unstained table cloth in a peasant home (partially because white is a lot of work to achieve and keep without modern bleaching and detergent).

The issue mostly comes down to a consistently clean and pristine presentation, even in places where that level of cleanliness would have been impractical to achieve and maintain. A castle that has seen battle will have some cracks or chipping in towers, maybe missing some "teeth" (merlons) in the crenellation that might have been replaced with makeshift wooden cover and never fully restored, you get the idea. When everything looks like freshly built, it just strains credulity.