Because it trivializes lockpicking. What's the point in a thief upping their lockpick skill if a mage can up their Alteration skill and do it more efficiently (no possibility of failure if you can cast the spell, no lockpicks, auto-replenishing magicka) and with more benefits (a whole bunch of other useful alteration spells)? Bypassing locks is a thief skill, not a mage skill.
Makes map design harder when the player can just go flying freely in any direction at almost any time, and leaves combat exploits wide open.
Invisible does not mean silent.
I think there are enough exploits available through enchanting and alchemy which more than out weigh any expliots having levitate would bring. As for lock picking, people who like the mini game and the feel of being a thief would still use lockpicking and would not have to waste skill points and perks leveling up a magic related skill if they have no intention of using magic for other things. You had to put a substantial number of skill points into alteration in oblivion to open the big locks. Big waste for a thief to do that just to avoid lockpicking. All in all, removing fun elements like levitate ruins what elder scrolls is all about, freedom. Elder scrolls has been and always will be easily exploitable. Remove levitate, but not 100 percent chamelion armor? See my point?
Also levitate does not mean super man fly, there are limits and mana consumption to balance it.
And finaly, the amount of humiliation one must feel as a mage, confronted with a lock and being forced to pull out a rusty lock pick.... can not be explained on this forum post... He can devestate armies with a wave of his hand, but a lock confounds him.... Sad day.