This is one of those things you should probably just take on authority, like relativity or the standard model of particle physics. That is to say, it's an exceedingly complex topic in practice, and any argument stated for either side which can readily be understood is likely to be wrong. You have two or three options: study the field long enough to know what's going on, or trust the people who have already done so. (The third option, 'form an opinion without having any idea what's going on', is also commonly taken.)

In short: I believe it's happening because this is what scientists tell me, and it's not worth putting in the time required to understand the field well enough that I could trust my opinion over theirs.