Random Opera

I can't seem to win with this one, but I guess I can give it a shot
The people who haven't seen this opera will be confused
The people who have, no doubt, will tell me something along the lines of that's not what happened. I took Western Civilization, I know what happened!
I am of course talking about....Verdi's Romantic opera called Aida--big R Romantic as in period in art history and music where for some reason people like to paint creepy depictions of the greek titans (thank you Goya)and write nostalgic compositions about the good ol' days of the French Revolution (Beethoven's 5th, anyone). I am not talking about the little r romantic that of course connotes a sort of sugary sweet misconception that rarely happens outside a movie starring Drew Barrymore.
Anyways, here's the very VERY abridged version of Aida. Egyptian boy (military commander acutally, but let's not split hairs) meets Ethiopian girl (princess, but ditto). Egyptian boy enslaves Ethipian girl as per command of the Pharoah. Egyptian Boy falls in love with Ethiopian girl (oh, did I mention she is Aida? no? well, better late than never). Egyptian boy also loved by Pharoah's daughter, but alas, this love is completely unrequited. Egyptian boy accused of something unspeakable, condemned to death.
Ok, here's what I dont get. When the Egyptian, Radames, is sentenced to death, he is locked up in a tomb. However, the tomb is not sealed. Aida manages to slip the guards so she can at least die with her true love. They then proceed to die together in the tomb. Big Question: If she could sneak in, HOW COME THEY BOTH COULDNT SNEAK OUT??!!
This has been plagueing me for almost a year. If you have an answer, or want to call me a freak or something (of which I'm already aware by the way), by all means, speak up. My best friend will also greatly appreciate this, as the question arose during one of our many, let's say, off-topic conversations.