Numerous reasons.
Firstly uneven performance is to be avoided.
Especially if you have oe bushes, this doesn't apply to poly. If one oe front bush fails and is replaced tother will be softer, so given the level of toe out on the brakes designed into the bush movement you could get the car bearing off under braking or emergency braking.
Rearward bush also but less likely as there is less give in that one.
Then there's the set up issues as Kev says.
Changing bones means set up after. Changing one means set up. Then set up again when the other fails soon after. Plus you've got all the tools out, you may as well do both.
But the main reason is to give even performance across a given axle. It's the soft squidgy bits that need to be even. Bush play on bones. Spring rates and damping rates.
Oil and gas in the shocks. Etc.
Drop links don't really apply to the same level as the play just causes an annoying knock. It doesn't affect performance to have 0.05mm of play in one drop link. Only thing to consider is tother side won't be far behind on wear. There no set up either obviously.
There's owners preference to consider. Most people don't want the agro so may as well do drop links while doing bones, is one line of thinking. I wouldn't change them unless they knock personally.
But in short. Reason is, even performance across a given axle.