I think it's a media attention thing. Flights are overbooked all over the world, and people get involuntary bumped. I just did a couple of searches for "[lufthansa, british airways, qatar, emirates] overbooked", and it's not exactly unheard of. It's a relatively rare event, though, but one that caught US media attention due to the pretty gruesome removal of Dr Dao last year (which wasn't even an overbooking incident).
There's also the factor of the uniquely dense and complex nature of the US domestic network. More flights with more connections make for more uncertainty in the system, to which overbooking is an answer.
"Flights are overbooked all over the world" Yes, but overall at a far lower intensity than in the US. I don't just mean 55/45 type things: I have flown continuously internationally for the last 20 years, worldwide, about 5-10 trips per year, and the only economy I have been bumped in was the USA. The data you gave about search matches in no way addresses the huge skew to events in the USA.
Contrariwise, the UK is the only economy I have used trains in, which routinely has major commuter train cancellations. The commonality here is a lack of capital investment and oversight
uniquely dense and complex nature of the US domestic network. More flights with more connections make for more uncertainty in the system, to which overbooking is an answer Yes. this I think is true. the US has far more dense flights, and has chosen overbooking as an answer but in no sense is it the answer to this problem: It's the one we've got. The regulator could have directed other approaches, but the cost consequences on airlines (who routinely seem to use chapter-11 methods to walk away from historical debt) would have been pretty bad. This answer maximises profit for airlines, at the expense of flyer convenience.
There's also the factor of the uniquely dense and complex nature of the US domestic network. More flights with more connections make for more uncertainty in the system, to which overbooking is an answer.