That's because the point of the joke of "Infernal" and "Big Lablooski" involved them in areas NOT in the Foster's house. Both those episode took place in locations not evident in the foster's home, where this one did.
Bloo and the others snuck out, they explained that, they didn't bother saying where Frankie and Herriman was when all of this crap was going on. And believe me, if they were home they would have DEFINITELY heard all of that noise.
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