Just been ruminating on it... I think my official head canon:
In the MCU, Odin must have known about Tiamut. That's why he hid the Tesseract on earth, because no one was going to come here looking for trouble. Meaning, the easiest answer for why the MCU is so drastically different is that the celestial seed was never planted on earth.
Asgard may protect midgard from Frost Giants (leading to Norse mythology), but Odin doesn't feel comfortable leaving the Tesseract in the care of the Earthlings without celestial protection. Red Skull was probably tipped off at some point in the early 1900s about the Tesseract. Which allowed him to rise to power and fund Erskine's formula.
So far this negates 8 movies (Thor, The Dark World, Ragnarok, The Avengers, The Incredible Hulk, Captain Marvel, and Eternals)
No Red Skull means Hydra is never formed, SHIELD is never formed. No Shield means no Pym. If the pym particles exist, they are not public.
Bucky was never brainwashed and he never killed Howard who probably made amends with Tony who in turn ended up with far fewer demons. He may be in weapons manufacture to some degree, or heck, maybe just worked on cars. Either way, Ultron remains a twinkle in his eye.
This negates 9 more movies (Iron Man 1, 2 and 3, The First Avenger, Winter Soldier, Age of Ultron, Ant-Man, Ant-Man and the Wasp, and Civil War)
Wakanda exists but remains hidden. Sokovia is never destroyed so T'chaka never leaves Wakanda to sign any accords. Kamar Taj may exist, but so much has changed, who knows if Strange crashed his Lambo or not. Perhaps the Time Stone is just a really well kept secret or also isn't on earth due to lack of celestial protection.
Vision probably doesnt exist, but Wanda likely does in some capacity (as she is a Nexus being... the mutant gene may be dormant in Sony's Spider-verses)
Thanos probably never comes to Earth in either Raimi's or Webb's universe. But if he does he probably has a focused assault on Kamar Taj and loses against TAO.
This really only leaves us with Guardians of the Galaxy... it's hard to say whether Quill left the planet or not. Things may get cleared up about this in the future. I speculate that Ego was not actually a celestial. That he was either a fraud or a fool and that might have something to do with it.
Anything beyond that (Disney+, etc) is the ripple effect. Enough of the 20th and 21st century changes that nothing happens the same way.
But Spidey remains. Peter Parker always gets bit by that spider.
