They don't look at prior art. Otherwise, they would have found all the prior art that the "vigilante engineer" found (and it's not like it was very hard to find, some was ironically linked from the patent itself).
They do look at prior art but remember they only have basically a "google search" type of tool so often find so-called duplicates that are not (because they have the same words in the title maybe) and miss ones that are the same because they are called something else.
Just a very poor system for the money it costs and the potential outcomes to businesses who can literally go bust over a wrong decision.
