Because the number of first-year quarterbacks who have come out in the last 15 years to be more prolific than Dak right away is 0. It's a 50-50 shot if a first-round QB turns into a starter, let alone a prolific one.
You can't give a rookie one year, they get 5. The math is very simple if you cut Dak after this year, even as a post June 1. Dak's cap hit in 2024 and 2025 is $25.4m. Assuming a rookie is taken at #10 overall, the cap hit will be ~$4m (J. Fields 3.4, G Wilson 3.7). So, you're effectively spending $30m against the cap on the quarterback position - that's significantly more than Dak has ever counted against the cap and doesn't give you any savings.
In your scenario, you're not getting any cap savings, you've got maybe a 25% chance to get a QB as good as Dak in the first two years, AND you're giving up a ton of draft capital to move up high enough to get a blue chip type of prospect. That only makes sense if you're rebuilding, not trying to win now.