While the challenges of miking an acoustic piano are real (I may have to try double-miking myself sometime), I must disagree with your statement that an electronic keyboard usually sounds better. I don't think ever heard a keyboard, through a sound system, that I could have fooled myself into thinking was a real piano, particularly if it's played across the compass with a wide range of tone colors. As a pianist, while I've encountered many out-of-tune or poorly-regulated acoustic pianos, there is simply no contest in terms of touch and subtlety of sound between a keyboard and a good piano.Listen to the piano sound on these clips . Ideally with good speakers or headphones. Some of them are Amy playing the (good) baby grand in the Glen Echo Ballroom while others are on my relatively low end Yamaha P85 keyboard, in random order: check the key.
I think the keyboard and real piano both sound good, but I do think the keyboard sounds a little better. Modern keyboards work by playing back lots of high quality recorded samples made off of a very good piano, so it's not surprising that they can do a very good job at the piano sound.
 These are clips I generated when trying to figure out if we'd gotten better over time. Look there for details on how I made them. I've tried to select here ones where the piano can be heard well, instead of being drowned out by too much mandolin or something.