Why are all authors credited equally with authorship regardless of level of contribution?
My opinion is that, contrary to what you say about "one main contributor", in most cases, it is absolutely impossible to judge "the level of contribution" from any objective standpoint. Let's consider a (not hypothetical, just missing real names!) example.

A and B spent about a year thinking of a problem and devising a scheme for solution they could not make work.

A discussed the question with C.

C, who wasn't much interested in trying it yourself, passed it to his collaborator on a different project D.

D found an approach that gives fairly good result but not quite what was wanted and told it to C.

C passed it to A and B and during the two week visit of D arranged that A,C,D have a few discussions about it that resulted in some extra ideas but not a full solution yet.

Meanwhile B tried to combine what he knew himself and what he was told by C and obtained an even slightly better result than D (though still short of the exact statement wanted) but under more restrictive assumptions. He sent it to D.

D, upon reading B's draft, realized that the initial scheme of A and B could be made to work after all (what he was missing was in that note from B and what B was missing was a part of D's "general knowledge"), finished it off, and sent the solution to A,B,C, who have read and verified it.

Now, I suggest you try to tell the "level of contribution" of each person keeping in mind that

1) If not for C, D would most likely not hear of the problem at all and it is doubtful that he would come into direct contact with A,B.

2) It is possible that A and B would make their approach work eventually without D.

3) What was a "general knowledge" for D and allowed him to finish the problem off, would hardly come into the mind of A,B,C at all.

4) Without B's draft, D would, most likely, stop at the "partial result" he obtained first.

5) Both A and C participated in the discussion during D's visit and, while everybody remembers all the ideas that surfaced, nobody remembers (or cares much about) who said what. 

6) A,B,C,D all argue that the rest 3 could surely do the problem without him, just in longer time, so the current idea of "author credit" C and D have is to publish the whole thing under the name AB CoD.