[UrLicht]

Compared to pianos, violins are really, really simple machines. You have strings, a bridge, a sound box with some holes in it and a couple of posts inside, some tuning pegs and that's about it. A single key on a piano has way more parts, and most of them move.

The design and construction of string instruments like violins is pretty much a solved problem. Yes there is variation in the way the wood is shaped and carved, which gives you the different grades of quality, but the basic engineering problem of "how do you make a violin" is solved.

Because a piano is much more complex there is a lot of room for continual innovation in both materials and construction. This might be the reason that you don't see people performing on old pianos - the newer ones are just technologically superior. That and it just takes so much effort (and money) to maintain a quality piano.

That being said, there is a legion of both pianists and violinists that will tell you (incorrectly or not) that the sound of an instrument improves with age. Something about the fibers in the wood being affected by vibration.

[Edit: maintenance cost as a factor]