I can't answer directly the titular question, but, having served in the organizing committee of a couple of conferences, I can try to explain the high conference fees.
Let's take as example a medium-sized 5-day conference with about 500 participants. The main expense is the venue: a quality venue with a capacity of 500 participants, with several rooms for parallel sessions, can cost around (200-250) k€ for five days, probably more for high-quality venues in several countries (e.g. large 5-star hotels in the US). This makes already (400-600) € per participant.
Then, you have to serve lunches and coffee breaks. This, depending on the service, can cost around (20-30) €/day. For five days, this adds another (100-150) € to the conference fees.
Then there are the conference gadgets and the proceedings. These are not too expensive if the proceedings are edited voluntarily by the organizers -- let's consider around 50 €/participants for both the gadgets and the proceedings.
Now, what about inviting a few speakers? Paying the travel, lodging and conference expenses of, say, five speakers can cost around 20 k€, which divided among 500 participants, adds another 40 €/participant.
Finally, there is the support for young researchers, and this can add another 50 €/participant.
So, we are already in the (700-900) € range. But, wait, am I missing something? Sure, how could I forget? Of course, you might need to pay some extra-personnel, depending on how many services you need to outsource (conference secretariat, side events, etc.). These can quickly add another bunch of euros for participants. And if you also want to the social dinner, add another bunch.
You see that we have quickly arrived to conference fees in the (800-1000) € range.