A piano is not a neutral instance; a score for piano is not written on a blank slate and cannot be arbitrarily transferred to other resources, without loss or accretion of information. Such additions and losses may or may not be useful, interesting, or musical, but they are certainly compositionally provocative. A score "reduced" from an ensemble of voices or instruments is not necessarily an interesting piano piece, but it may raise interesting musical issues which could lead to an interesting piano piece, particularily regarding questions of "orchestration" for the piano: registration, texture, polyphony, doublings, etc..
Curious thing, that the piano is often a default setting for composition. Yes, composers are often piano players, and yes, the piano can do some things very, if not uniquely, well (e.g. attack dynamics, some polyphony). But too often, we either forget or ignore the uniquity of the instrument, its unique specializations: its temperament and "stretching" of the tuning across the range of the instrument; the radical timbral differences between registers; the unmistakeable attack and decay; the various technical kludges that can be used to suggest — among other things — a physically impossible degree of sustain; the complex interactions among the ensemble of wires, particularly sympathetic vibration; the fact that most pianists play the instrument available for a concert — a found sound, if you will —, rather than bring their own, the one with which the relationship is most intimate.
But then again, having some form of a blank slate is VERY useful. The "open score", for example, is a way of gathering material while suspending judgement on the ultimate assignations of the material, keeping options open to both practical circumstances and ideals, exploring the plasticity of the material, allowing the material some productive promiscuity. Blank & open, yes, but not yet necessarily a piano piece.