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Why aren't all frames designed for inline posts? Why are most posts layback except Thomson which are almost all straight?
I don't know about the first question but the second is because you can slam the saddle down with a straight post.
They have layback so you can sit in the correct place for maximum comfort/leg power.
Different people have different length legs (femurs) so may/may nor require a layback seatpost.
I have a Syntace seatpost which is great cos its inline but also offers quite a bit of layback.
I think brant designs around inline posts. Most layback is done in the head not the post - thomson are one of the few that do it in the shaft.
I don't think most frames are 'designed' for one or another - except for custom frames when it should be taken in to account. In theory a layback post allows you to use a steeper seat angle and keep chainstays short but still keep the pedalling position reasonable.
As to the initial question - some don't and others have varying amounts of it. It allows you to fine tine bike fit in the same way as stem length etc.
I'd imagine it's because before we had micro adjust posts with integrated clamps, the saddles clamp positioned the saddle just behind the post, and so micro adjust posts mimicked this position.