It can't be because he has no form. Though post biblical interpreters for the most part* explained away their implications, the Hebrew Bible has dozens of verses which suggest strongly that God appears and can be seen:
- Then Solomon began to build the temple of the LORD in Jerusalem on Mount Moriah, where the LORD had appeared to his father David. - 2 Chron. 3:1
- At Gibeon the LORD appeared to Solomon during the night in a dream... -1 Kings 3:5
- The LORD appeared to Abram and said, "To your offspring I will give this land." So he built an altar there to the LORD, who had appeared to him. - Gen 12:7
- My soul thirsts for God, for the living God, for the time when I can go and see God -Ps 42
- You, O LORD, are among these people and you, O LORD, are seen by them eye to eye... - Num 14:14
- They saw God, and they ate and they drank - Ex. 24:11
- ...no one may see me and live. -- Ex 33:20 (implies rather strongly there IS something to see, only the experience of seeing it would be fatal)
If you're a Jew, with a an education like mine, you "know" none of these verses are to be understood literally, yet, it must be asked: If God, his secretaries and spokesmen were all able to speak so casually about Him having a form, why couldn't that form be depicted in art? We don't mind the verses that speak of a corporeal God because we all "know" the verses don't mean what they seem to mean. We read them with something like a wink. If we can make that mental correction when it comes to verses, why can't we make the same mental correction when it comes to statues? If I know (thanks to my fine education) that the Written Torah depicts things that aren't literally true, couldn't that same education teach me how to regard statues of God? Why was banning them necessary?
An answer, maybe, will appear in a later post.
* For the most part... Philo says Israel is called Israel from ish ra'ah El, or "a man seeing God". God also takes a human form in a great many aggadot, and there is reason to beleive that Rashi, among other Rishonim, were corprealists, too (or at least that they didn't consider it heresy to hold this view of God.) Deeply uncomfortable with these facts? That's because the victory of the Geonim and Rambam on this point was a absolutely complete.
Search for more information about the second commandment at 4torah.com.