Why does that happen to cars? How can i prevent it from happeneing?

Theres a small patch on my rear one, wondering how i can treat it before it gets worse?

The rust has crept in under the paint. Need to rub down to bare metal 1 inch around the rust edge. Treat with "kurust" then repaint from scratch. Dont forget to treat inside the wheel arch. Remove any liners as needed. Check the metal . Rub down any rust & treat. Then a coat of paint + 2 coats of underseal.

It's worse if you don't wash under your arches properly and regularly too. The mud gets in and slowly starts the rust off if it stays too long.

Ferrous oxide - the result of steel being converted to disheartening dissolvement via chemical reaction.

If the steel is sealed from things like water and oxygen,its likely to stay useful.

Its no good sealing in the rust though,as it'll grow within and push its way out (bubbles in the paint). Chemicals are available to convert it though,such as Hammerite's Kurust. Wire-wheel the paint and surface rust off,apply,re-paint/seal. But it cant work miracles,and if the steel is perforated,it'll have to be replaced with good metal.