4 Answers

Like most thing it has to do with MONEY. It is cheaper to produce thousands of "rubber" backs than the metal or gold ones to secure earrings on.

it also have something to do with skin reaction either alergenic or just discomfort.

Actually, I found that the rubber earrings are more stable than the metal or gold 'butterfly' backings.
It all comes down to how 'thick' the metal or gold rod is, as these rods vary in width. (rod being attached to earring and goes thru ear). I have lost so many earrings, I now have many unmatched pairs.
That is how I discovered that the new rubber backings are more stable...I don't lose earrings anymore.
(touch wood)

obviously someone has sussed out that you will have at least one gold set so that you can alternate the stud to any earing hence a money saver for the manufaturer