The main point is the Ark does everything.
The Ark is the prime actor in the film.
The Ark saves Indiana Jones. He does not save himself. Not looking at it is hardly a heroic act. It's defensive.
Lucas said it himself--the problem after Raiders is that there is nothing as visually exciting as seeing these angels flying around and melting people.
Why couldn't he make as many Indiana Jones films as Sherlock Holmes, James Bond, or Doc Savage?
Because in addition to the fact that it had no single author for the character, the character is not as dynamic. He was created as a stock serial adventure hero (or anti-hero actually) to serve a purpose and the treasure quest is the gimmick.
I don't feel like we've seen the same film.
The ark is a plot pivot, not a character. It doesn't actively do anything, has no personality and accomplishes nothing. It isn't even mentioned until 20 minutes into the film.
Indiana Jones is a dynamic figure that is revealed like an onion throughout the picture, starting out as an apparent grubby villain, then womanizer, and eventually a national hero. He is funny, interesting, smart, skilled, brave and the best in his field. We see a full range of emotions from him, despite the format of the film. He ends the film as a very different person than how he started for the audience, experiencing the kind of character development that good fiction is all about.
Raiders overall is about as entertaining as filmmaking gets - scenery, stunts, story, character, humor, dialogue, music, effects and even style. It doesn't matter how the film was conceived or written - it only matters what ends up on the screen. Harrison Ford is excellent. The film is excellent. You can talk about its context as the first in a series or as the revival of an old tradition, but the film is the film and it stands on its own as a cinematic masterpiece.
The reason the other films aren't as good is that the first film transforms the MC in a way no sequel can do again. You can only pull that stunt once. That's the difference between writing a great story and writing a character designed for serialization. Holmes does not start with a fascinating introduction as much as a complete character that remains consistent as the stories go on.
And just to address the common complaint that Jones doesn't accomplish anything, this is completely untrue. The Nazi's plan was to recover the Ark and fly it directly to Germany. Once there, even if it killed the people that opened it, it would still be in Nazi hands - bestowing whatever powers upon them. It was only due to Jones that the Nazi's ended up with it aboard a submarine, putting it on course for a stop on a mostly empty island. Due to Jones' persistence and knowledge, he is in position to survive the opening and get the US to rescue the ark from the sub base before the next German sub pulled in. How he did that we don't see, but it would have also required more savvy and bravery.