Windowpane test - Why does my dough fail it, and what is it good for?
I had this exact same problem for years.  And it was all about letting the dough relax.  I'd get beautiful crusts, but never EVER pass the windowpane test.  I was so confused.  

Turns out, all I had to do was leave it alone for about 10 minutes, much less and I'd still run into issues.  The window pane test is intended to show that enough gluten has been formed that you can stretch the dough.  It's a completely non-exact test.  It's just to ballpark if there is 'enough' gluten.  By leaving it alone for 10 minutes or more, the gluten has relaxed and can be stretched to form the window pane.  

This relaxing is the same thing that you may need to do while forming your pizza.  If you find that your pizza dough will stretch out some, and then pull back constantly - its good because you've probably got enough gluten.  Let it relax for several minutes and then pull it back - go too far and you'll likely accidentally form a window pane!

Enough gluten means that you've formed an efficent 'net' of gluten that will trap gases from the yeast and also provide some 'chew'.  You'll get better rise and your bread will have a better crumb.  Yeast breads without much gluten development are often denser.

When hand kneading, an autolyse will also help (well, it always helps, just especially when you have to do the work manually). By giving the dough a short rest after initial mixing, you let the dough fully hydrate and some gluten will start to form on its own.  When you come back to knead the dough, you should find the dough more workable and a little easier to get to the 'kneaded' point.

 What's going on with 'relaxing' gluten?  Kneading forms gluten, but it often also tangles them up a bit.  With a short rest, they untangle a bit and will pull out into long strands easier.