In the MCU it is shown lines of men enlisting in 1940, including Bucky. Why was this when the US didn't join the conflict until December 1941?
I assume you know all this, but just for completeness, World War II started in 1939, and most of Europe had been preparing for something to happen for years before that (since it became clear that appeasing Hitler was a failed strategy).
The US officially did not join the war until after the attack on Pearl Harbor, but literally no one believed we were going to stay out of it. Even the official neutral position of the US government was crafted to allow us to aid the Allies in non-military ways (mostly by selling goods to them). Popular opinion in the States was that we were on the "side" of the Allies, and lots of people were looking forward to going out and beating up Hitler. Remember, at this point, war was still a mostly-popular occurrence for the US, as we'd just recently come off WWI and hadn't lived through Vietnam yet.
By 1940, the US had started ramping up its military presence, especially the navy (one main reason Japan attacked us was because it was pretty obvious to everyone that we were going to get sucked into the war sooner or later, and they wanted to get the first hit in) but also the army. The draft also officially started around this time, so even people who may not have been gung-ho about joining the army were getting drafted anyway.
Once the call for enlistment went out, people lined up to join the Army, for any number of reasons: some believed it was the right thing to do, some wanted to be heroes like their fathers and grandfathers, some just wanted the guaranteed pay and education the military promised.
But in general, it would have been perfectly natural for someone like Steve Rogers to try to enlist even before we actually went to war.
Some reasons I found why he would enlist before the war:
As from Marvel Wikia - Cinematic: Before Rogers was born, his father Joseph was confirmed to be Killed in Action in World War I. In 1940, Rogers, now nearly 22, discovered his mother died of tuberculosis. This points out to the fact he is all alone by now.
Also: Rogers and Barnes learned during an art class that the United States of America entered World War II. Rogers wanted to join the United States Army to fight in Europe against the Nazis, so Bucky trained him for two weeks at Goldie's Boxing Gym. Soon Bucky and Steve went to a US Recruiting and Induction Center in New York City where Steve was classified as 4F and rejected from service, but Bucky later enlisted in the Army.
While watching the Series Band Of Brothers, I noticed the following statement.
Richard Winters: These men have been through the toughest training the Army has to offer, under the worst possible circumstances, and they volunteered for it. 'Buck' Compton: Christ, Dick, I was just shooting craps with them. Richard Winters: You know why they volunteered? Because they knew that the man in the foxhole next to them would be the best, not some draftee who's going to get them killed.
Also, IRL America just went through the 'Recession of 1937–1938', part of the Great Depression. This meant no jobs or income. Better the reason to enlist.