It's the same reason Fox's Quicksilver wasn't in WandaVision, despite every single clickbait website and Reddit/Twitter bullshittter claiming he was.
The way Hollywood works is that the Producer of a superhero movie owns the versions of the characters in those movies. So if Feige were to use Bryan Singer and Simon Kinberg's version of Quicksilver they would have to be paid and given a Producer credit on WandaVision.
There is a cosmic epic level of bad blood between Avi Arad and Kevin Feige and part of the deal with Sony for the MCU Spider-Man movies was that Amy Pascal would be in charge on the Sony side and Avi Arad and Matt Tolmach wouldn't be allowed anywhere near the Spider-Man solo films. If they were to reuse the Avi Arad versions of the villains from those movies, then Fiege would have to pay Arad and give him a Producer credit in the MCU. I'm sure Avi Arad would love that, but Marvel wouldn't.
He was bullshitting in that interview about the river thing (like when Michael Shannon said he had flippers on his hands for Zod mocap). It's the same villain, but a different version of it from an alternate universe.