Just a slight correction, the Apple Watch sport bands are not made of "Rubber", they're made of flueroastamer. Entirely two different materials. Given they are expensive what Apple charges, third party bands don't even compare to Apples OEM sport band material and construction.
Here is some history on Flueroastamer:
Fluoroelastomer comes from the two words "fluorinated" and "elastomer". Fluoroelastomers are famous for their durability resistance to heat, oils, solvents and fuels. They are often used for fuel-distribution systems or for high-quality lab gloves. It is not a material that will stretch out easily, get damaged by extreme heats, or get deformed by being excessively sweat on. It is also much less prone to induce any allergic reactions than, for example, latex."
Where as rubber, doesn't have the consistency and durability as Flueroastamer does.