I think there are a lot of reasons, but I first want to address the one that you're probably most likely to have heard of, because actually it's more dangerous than you might think.
Manuprāt, iemeslu ir daudz, bet vispirms vēlos pievērsties tam, par kuru visticamāk esat dzirdējuši, jo tas patiesībā ir bīstamāks, nekā varētu likties.