According to this page http://www.computerchess.org.uk/ccrl/404/ http://www.computerchess.org.uk/ccrl/404/ Stockfish has the highest ELO of all engines... but

According to this page http://www.computerchess.org.uk/ccrl/404/ http://www.computerchess.org.uk/ccrl/404/ Stockfish has the highest ELO of all engines... but how can an open-source engine be best? How don't the other engines learn from it and become at least as good, if not better?