When people make arguments like "taxes should be higher" or "air travel should be more expensive", they almost always mean "for other people".
People need to eat their own cooking or they instantly lose all credibility, even when the facts are on their side.
Also, it’s really hard to live in the modern world without using fossil fuel-based comforts. You can criticize air travel and still travel to a climate conference. It doesn’t make climate change any less real. You can maybe argue that people are hypocrites, but so what? The ice caps don’t give a shit. And maybe that meeting of scientists in-person is what it takes to fix a really big global problem. Flying to Asia every year for a vacation definitely won’t do that, though.
Every Hollywood actor demanding we take action on climate change while flying around in a private jet weakens the argument for real action on climate change. It makes the issue look like a vanity cause even though it's not.
You see the same thing on taxes. You've got guys like Bernie Sanders out there demanding we all pay more taxes while he himself has a very low rate of charitable giving by most standards. He could also opt to voluntarily pay more taxes right now. Of course he doesn't, which again undercuts any broader point he may have.
Human beings are hardwired to be turned off by hypocrisy.
It could be viewed as an investment in future lower emissions, the same way spending money on R&D today can improve profits in the future.
Even voluntary and cooperative solutions would benefit from the type of coordination an in-person conference affords.