Why are tweeters so high off of the ground in many tower speakers...

Depends on the radiation pattern. One assumes the on axis response of the speaker measures or at least sounds flat when the listener is seated. There are so many room acoustics factors that complicate frequency response of any speaker as to almost make the question mute. Oops, I mean moot. 

With your loudspeaker at that height the accuton tweeter is off axis pretty much bouncing its energy over a seated listeners head that is sub optimal design. Design appears to be modeled after the old Wilson watt puppy setup and that I have had experience with good for standing listening seated not so much so.

I needed to research your speakers before chiming in.... Well, these have 1st order slopes and the rolloff is very minimal.  Unless you are fairly far away from your speakers,  pointing them downward would slightly increase the frequencies of both drivers around the crossover point. They would most likely sound peaky. 
Normal ear height is 38 to 48 inches,  when I build,  I design for this height. Also,  you want the sensitive frequencies of 800hz through around 3k centered at your ear.  These frequencies on your speakers are all coming from your woofer.  So if your ear height is 40 inches,  I would want the top ring area of my woofer also at 40 inches. Hopefully,  you sit 7 to 8 fit from your speakers and your speakers are around the same distance apart.   So,  I would start by adjusting the height of your speakers so that your woofer ring is 40 inches,  I would then point them in full tow,  meaning that your speakers point directly at your ears... Listen for sound stage imaging and any peaks in frequency.  If anything is not right,  slowly start turning your tow outward,  just an inch or so at a time, until you find the best overall balance.... 
I gave you a couple of ifs,  so if I am off from how far you sit from your speakers and how far your speakers are apart, please let me know and I'll re figure this based upon your listening position.
I hope this helps, 
Tim

random tail chasing, marketing and a lack of real engineering skill combined with a lack of enduring principles ?
i always get a crackup out of an 8' tall Diana Krall singing a stinging Case of You like she is standing on 6 cases of vino....

i will say measuring listening position ear height and a few other very important variables is key to a properly setup pair of Vandersteens and yes tilt is important.