It's that the marginal benefit of individual action accrues mostly to other people, or (on a national scale) to other nations. The fraction of benefit that accrues locally isn't enough to justify the cost (unlike, say, the ban on CFCs, or control of local pollution.)

So absent something enforcing prohibition of defection from a collective action, the collective action doesn't happen.

You want to actually solve the problem? Find such an enforcement mechanism (CO2 tariffs, perhaps), reduce the cost of solution (sufficiently cheap non-fossil energy), or find another solution that doesn't require global cooperation (albedo modification, say).

A solution that just requires everyone to get along and cooperate to their marginal net detriment doesn't seem like it will work.