Don’t blame Nielsen.
Compared to, say, curing the common cold, it’s relatively very easy for Nielsen to measure out-of-home viewing. But easy does not equal “cheap” or “free” so more accurately one could say “it’s easy, but expensive to measure and nobody wants to foot the bill.”
Lots of World Cup fans wish Nielsen measured ALL viewing. But Nielsen doesn’t exist for World Cup fans or fans in general. It exists to facilitate advertising.
Nielsen has tried (tried, and tried again) with out of home viewing measurement. It has found things that worked pretty well. But, not surprisingly, advertisers don’t care. Advertisers don’t want to pay for people at bars, parties, etc. and the TV networks weren’t able to move them to part with their cash to pay for those viewers.
The reason why out of home viewing isn’t measured isn’t that Nielsen sucks or that it’s too hard to figure out, it’s that advertisers don’t want to/aren’t willing to pay for those viewers, so nobody is willing to pay Nielsen to measure it.
Nielsen isn’t a charity, so with nobody willing to pay for such measurement, it doesn’t measure it.