A few facts and possibilities:
1) UFO aren't always 'flying saucers' type. There has been many shapes of cigar/cylinder shaped objects and other with lightnings and fireballs with no definite physical appearence. There are videos and photos on the web of some of these.

2) Few reason they don't make contact and observe only:
i) Ever watched Star Trek? You'll be familiar with their Prime Directive of non-interference with Civilization that is not yet capable of space faring. Maybe to them, we're still not there yet, thus they'll not show themselves to us yet.

ii) The US gov and other major powers are aware and in contact with these Extra Terrasterial Life forms. And are in technology and information exchange with them.

iii) They have evil intentions and plotting a sinister plot to destroy or enslave us all. Perhaps something is holding them back now...maybe the 10,000 Starship Class Destroyers is still 500,000 million light years away and they're just waiting for backup....