[patatino]

And who knows why I always have to search for the login link? Sure, new accounts are cool, but existing members should be treated better

[seattle_spring]

I can answer this one:

Companies optimize for getting new users. Highlighting the register button reduces some friction that may have otherwise prevented some users from signing up.

On the other hand, a user that wants to sign in is much less likely to "give up" if they don't immediately see the button. Important and limited header space therefore is dedicated to getting new users.