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For example, why couldn't brake fluid be used for transmissions or vice versa or power steering fluid as brake fluid etc with the exception of coolant
Aren't they all a form of oil that isn't compressible? Is it for temperature reasons? Seals? Why couldn't the same seal material be used for different systems if that's one of the reasons
The fluids are all designed for different conditions, more viscous so they "stick" longer, working temperature ranges, behaviour under pressure ie in the engine bearings etc.
Some of the early cars had hydraulic brakes - with water as the working fluid but in winter it was replaced with methylated spirits as it did not freeze... but it did evaporate rapidly. Water had the problem that it boils if the brakes get too hot so became the challenge of designing a fluid for the brakes that works in the conditions.
The brake fluid we have now is dimensionally stable over the working temperature range and, as a bonus it is an excellent paint remover...
