Because the filters are useless.

You greatly underestimate the breadth and depth of the prudishness that motivates the creation of content filters.

Except we're talking about a variety of filters. The filter in the op is an extreme form of filter that is perhaps useful for a locked down kiosk like environment. (Not as good as doing that properly, obviously) and so that filter is very broad, be because it wants to catch everything and allow owners to whitelist exceptions.

The ukgov porn filter will suffer from some of that, but also the Scunthorpe problem.

Having had blood test results delayed because my surname has the word COCKS In it was frustrating.