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In order to get Harry out of the Dursleys' house, the aurors decided to come up with a plan to use polyjuice potion to create seven Harry Potters, providing confusion over who the Death Eaters should go after. This was to avoid the problem of apparating out of the house, since Harry still had the Trace and the Ministry would know where he went. (Floo powder and portkeys also had some Ministry oversight, so were off the table).
Why didn't Harry just leave while wearing the Invisibility Cloak, perhaps going out at the same time that the Dursleys were moving out? That doesn't involve casting any spells and nobody can see Harry. So long as he's smart enough to get out without having any strangely opening doors, I think he'd be safe. Heck, since the Cloak is large enough to cover three children, perhaps he could even ride his broomstick while wearing it. Sure, there's the homenum revelio spell, but I can't imagine the Death Eaters casting it continuously for days on end.
Why didn't Harry leave the Dursleys' house under the Invisibility Cloak instead of having the Seven Potters plan?