Because while 2020 and the early part of 2021 can be written off in the eyes of Wall Street due to Covid and losses can be taken, Big Budget films still need to make money in order for them to justify making more of them. Disney+ already has MCU shows that they are paying top dollar for (and taking losses on), they can't afford to take losses on the movies as well. I think a 45-day window is fine, especially since big budget movies make the vast majority of their money in the first 2-3 weeks anyway.
Also, it's not like Shang-Chi or Free Guy will be streaming along with theatrical release. It just means it will be on streaming sooner than it would've been before. Disney has clearly shown that their blockbusters will only get dropped on Disney+ day and date through Premiere Access. Chapek must consider animation a lesser than product than he really should because that's the only thing he's been willing to just give to Disney+ for free.