The acoustic piano is a highly evolved, complex mechanical thing that is beloved in every culture. As good as digital keyboard equivalents have become, a few aspects of the originals remain out of reach:
Multiple key strikes/restrikes: When you play a key repeatedly on an acoustic piano, the previous note doesn’t stop ringing, and the vibrating string doesn’t come to a complete halt between strikes. The second (and third, and so on) strike of the same key in quick succession seems to add to the vibrations in a pleasing fashion and doesn’t sound like fresh strikes happening from silence.
String/note interaction: When more than one note is played at a time and the dampers are off those strings, the striking of the new notes being played cause the currently vibrating/sustaining strings to react in different ways and at different volumes.
Digital pianos and software are starting to try to replicate these behaviors, but they’ve taken only baby steps in this regard. The richer the music you play, using more notes and harmonies with a lot of sustain pedal, the more easily you can hear the difference between a good grand piano and the best digital equivalent.
Refined touch/control of dynamics: Advanced and accomplished concert pianists have spent decades working on their touch and control of dynamics on the best acoustic grand pianos. They can control not only the volume but also the tone of the instrument through their touch. And those skills just don’t successfully translate to the digital instrument.
Some feel you can get close, especially when you’re only listening to the performance. But for the player trying to express herself in such a skillfully nuanced fashion close isn’t close enough.
These discrepancies are why a serious classical performer or student aiming for a career in concert performing requires the acoustic instrument. Likewise, few top-class jazz pianists will ever be satisfied with a digital instrument for their recordings and live performances. Many may use electronics as an additional sound color and for studio purposes but not for their most important playing.
The piano isn’t the only instrument that brings players to this deciding line. Tthe tonewheel organ is a complex instrument, with a unique multicontact key mechanism and richly interactive sound generator. Every instrument (even the same model and year of manufacture) sounds a little different because of aging and care (or lack thereof), and many top players play only the real instrument.
The same standard holds true for clavs and analog synths; you still see an artist like Stevie Wonder performing with a real clav on top of his synth workstation keyboard.
And of course, appearance is everything. For many artists and bands, having the real instrument is a badge of honor. They want to use the original instrument to show they respect it, they have discerning taste, or they’re just cool. And that’s okay!