Think about it. A new civ gets introduced and everybody already knows how the bonuses work because other civs already have them, but this new combination of them leads to very different gameplay. It would add depth without complexity.
Take, for instance, “gold miners work x% faster”: what if Malians got it alongside Turks? Turks would still be a cav-archer or gunpowder hussar civ, while Malians would be an infantry cavalry civ. Or take, “Militia-line upgrades free”: What if Aztecs also got that bonus? Bulgarians would still have the cavalry focus while Aztecs could have more accessible longswords to complement their general infantry play.
There are many more situations where it would be great (mediocre examples above, notwithstanding) for a civ to have a particular bonus but another civ already has it.