That's actually not entirely true. Final assembly of the bomb was completed in flight en route to the target by the air crew. There were fears that if the Enola Gay were to crash on take off, the bomb might detonate accidentally and destroy the air field on Tinian. We now know that its impossible for an nuclear weapon to go off accidentally, as they specifically designed not to be able to do that, but considering that this was the first time that anyone had ever dropped an atomic bomb, they weren't taking any chances.
Actually, Little Boy was one of the few atomic bombs where there actually is a serious risk of nuclear detonation. Unlike modern implosion-type designs, where all the explosives have to detonate exactly at the same time, Little Boy was a gun-type weapon, where there's only one mass of explosive, and if it goes off the bomb goes off. If the cordite bags had been preloaded and the Enola Gay had crashed on takeoff, there's a serious chance that the crash might have set off the cordite bags, launching the projectile down the barrel and obliterating the airfield. Even without the cordite bags loaded, Little Boy, or any gun-type weapon for that matter, is inherently an extremely dangerous weapon. If a crash forced the uranium projectile and target together, which was very possible, the VERY BEST that could happen would be a massive release of radiation (which would also almost certainly happen if the Enola Gay crashed into the ocean, as the seawater would act as a moderator and let the uranium reach criticality, also causing a massive release of radiation and a large steam explosion which would contaminate a large area of sea with radioactive fission and activation products), and the worst would be a FULL-YIELD NUCLEAR EXPLOSION. Which was quite possible. In which case, bye-bye Tinian.