Since they are quite good opposites, the smart people who figured how all this worked decided on that and we stuck with it based on convention. Like how "Alice" and "Bob" were used in Computer Science since they are generic names beginning with the first two letters of the alphabet (it could have easily been any A and B name, but this is the convention!)

Similar can be said for magnets, the "North" and "South" poles are good opposites. If other people started the trend, we could have easily gotten something else, but this is the convention.

Guessing here, but +/- is good for describing them as binary opposites as that system already exists. This is a good thing assuming there are two types of charge/pole which behave in opposite ways (Eg move differently in a field). It's also just good to use numbers so that we can describe the amount of + and the amount of -, which numbers already do. It also allows us to describe neutral as neither + nor -, but 0. Again, we already have a scaffold there for numbers and it's easy to copy it for new things when that makes sense.

Magnets.

If there's no field there's 0, if there's a magnetic field clockwise it makes a positive charge, if there's a magnetic field rotating counterclockwise it makes a negative charge,

Likewise if there's a positive charge it makes a clockwise magnetic field and if there's a negative charge it makes a counter clockwise field. (I may be backwards +/- clockwise/counter clockwise, something about the thumb on my left hand..., but really it's all arbitrarily named, but the reason you just say negative or positive is that those are scalable measures, you can't have half a Bob or 2 Alice. )