War is expensive. War costs money, it costs lives, and it costs political capital. Prevention of war involving ones own territory is the top priority for any world leader.
World leaders right now are likely weighting the options of effectively dealing with this aggressive territorial expansion. While the opportunity for this to turn into a very similar situation to the beginning of WWII is high, it is not guaranteed, and prevention is still viewed as a viable option.
Letting Putin take Ukraine without direct conflict with other nations (which is what happening right now) and risk having a WWIII situation brewing if Russia moves to take another territory later is the safer route if diplomacy can be pursued to attempt to prevent further territorial expansion so long as it keeps other nations out of direct conflict.
Alternatively, Military force can be threatened, which would risk triggering a world war right now, but would theoretically end the conflict immediately (so long as it didn't escalate it). As it stands, negotiating to prevent (a larger) war is preferred over stopping the current war, simply because it's viewed as achievable, and includes less risk to the country not currently in the conflict (in this case, France).