Why do colors bleed in the wash ?

[Jumong]

It seems pretty rampant these days, whether it's polyester or Egyptian cotton. You can pay $8 for a T-shirt and the color bleeds in the wash. The same thing happens to a $50 shirt too.

What actually causes the color to run ? Perhaps, the more relevant question is in the past how did they manage to prevent the color from bleeding during wash ?

 

[Agoraphobic]

Garments sold in the past were "pre-washed?"

Cheers!