I've traveled to London several times, always via LHR, and have a pretty positive opinion of public transport from the airport. And I live in New York and can tell you, even as someone who takes public transport to the airport 95% of the time, that I would in no way consider the New York area airports' connectivity to the transit network to be superior to that of London's airports.
But I also think tourists/travelers, at least those who have a basic level of comfort with public transport, will often have a superior opinion of airport public transport options to locals. Travelers are more likely to be heading to destinations well-served by the transit lines to the airport instead of all over the region (they're heading to central London or the Manhattan CBD, instead of Sutton in London or Bay Ridge in Brooklyn) and are often coming from places that have vastly inferior airport transit -- I grew up flying out of the entirely road-based LAX so for a long term when I encountered any kind of rail transit to the airport I was impressed.
And I think travelers often have a different mental price calculus to locals, in that locals are often unfavorably comparing the costs of airport transit to their everyday costs for the "normal" transit network, versus travelers who tend to compare the costs of airport transit to a cab or car rental. I know that at home here in New York I grouse about the AirTrain fare ($5) because I'm annoyed that I have to pay more even though I already buy a monthly pass. But when I'm traveling from an airport abroad, I'm more often comparing the transit costs to the cost of a cab, and so (for example) any of the London options will generally seem a good deal for me.
Plus, of course, as a traveler you're usually either paying by the ride or buying some sort of daily/weekly pass for the rest of the network, both of which tend to work out to a higher cost per ride than for locals who will have some kind of unlimited/monthly/annual pass (as varies by the network), so then the airport transit doesn't seem so relatively costly either.