One word, globalization. This has led to a certain standardization of approaches to playing as opposed to in the past when there was more emphasis (acceptance) on a more personal approach. The same can be said of soloists on any instrument, not just violin and piano. On a “larger” scale the result of this globalization can be clearly heard in the major orchestras whose previously very distinctive personalities have become somewhat homogenized due to the phenomenon of the prevalence of the traveling guest conductor and, even more recently, principal players from different parts of the world who are the product of different “schools” and their respective approaches.
Btw, on the wonderful Milstein/Horowitz Brahms clip, I think that rv’s astute observation re tuning is, more than anything, a result of pitch instability in the playback of the recording. Obvious with the piano’s sound which doesn’t have the more forgiving “cushion” of the violin’s vibrato.