Shape shifting aliens.... Why?

 I was seriously bored the other day so I wasted an hour or so of my rapidly diminishing life by surfing youtube  Now theres lots of interesting stuff on youtube, and a few people I sort of follow who post some really good videos.... the trouble is theres a lot of... well shall we be nice and say crap on there too.
 Following one link to another I stumbled into one of these conspiricy websites, one about shape shifting alien lizards.
 Yes I know what your thinking, and your probably right.  I did watch the video and theres a few points the poster missed out.
 Lets assume just for a second that you are a shape shifting lizard (why are they always lizards?) now lets also assume that the shape shifting bit is a form of camouflage. Why then does the alien lizard always slip back into the lizard in front of Joe Public and give the game away?  It seems that as a camouflage it's pretty crap if you cant maintain it. 
 Besides theres a lot more to it than a simple shapeshift as camouflage, you'd have to mimic speach patterns and voice. Now there are a few creatures that can mimic the call of another, but as far as I can remember none of these has any ability to actually look like the other creature it's copying, and as for having a conversation and fooling the person your talking too.... You get my point.
 Probably the best "shapeshifter" is the octopus, these twist themselves to mimic things like a flatfist, and can change colour to hide, but only fairly briefly.
 But I suppose the clincher for me is way would alien shapeshifter lizards come here in the first place, to be honest most of the people I know want to leave this place, I can't think of any who would cross billions of miles to get here.
 By the way check out Tesco Port Talbot for shape shifting idiots.. theres loads there.
 And NO! theres no picture, and I really can't be bothered to make one.