Because the main function of the 3x recipe is to save time for crafters who AREN'T using the synth for xp. That's why it's there.. to save people time when making a bunch of mats.

Why is the 1x recipe there? Either to level, or people who only need one or two lumbers. Simple as that.

HOWEVER, I do agree... I would like to see the leveling process be less tedius and more challenging, or interesting even. Because right now, it just isn't. And that's kind of too bad!! At first it's kinda fun, but you only get new skills every 15-20 levels. Plus, like you said the most cost efficient, time efficient way to skill up is to just do easy synths that give decent xp. More complex crafts should most definitely give more xp. That would give crafters a real incentive to actually craft, say, lower level armor or weapons, etc. instead of spamming lumber all day.