I think we are talking about different things here? A shorter construction deadline doesn’t screw up staffing, training, or merchandise.
A shorter construction deadline means you are trying to accomplish the same big complicated task in a shorter amount of time. It's the same task, just less time to do it in. That means there's less room for error (hurricanes, strikes, setbacks of any type). You can't absorb contingencies. You are more screwed when bad things happen.
Buffering means phase A is given, say, 6 weeks even though on paper it should only take 4 weeks. So if there's a weather delay for a week, and it takes 5 weeks, they still meet the phase A complete deadline. Phase B start is unaffected. But it also means the site may look like nobody is working on the project for a week or two.
When you have a tight timeline, and only allow 4 weeks for phase A, and so phase B can't start on time, that subcontractor may take other work instead. They're not just sitting around waiting through whatever delays you have. They've lined up a project after yours.
If you set June 1, 2027 as your opening date and you hire employees starting April, with training set for May, and you don't actually open until September, what do you do with the employees, or the employees you hired to hire and train the employees? Put them furlough? Lose them to other work. Scramble to replace them?
What about the merch you told the Chinese factory you need to start flowing into the distribution center in April? The products start piling up because you're not selling them to guests. You start having to ask factories to hold off on production, but you committed to use their factory for a certain time period -- they have toys for Mattel and Hasbro scheduled next, you need to use the factory capacity you contracted for now. So you need to ask the factories to store the finished goods in China or rent shipping containers and put them in parking lots. It's a nightmare. It's expensive. Neon colored T-shirts you ordered for a Summer opening are not as useful in Fall when longer sleeves and muted colors are more appropriate.
Marketing bought advertising time on TV that is no longer useful. Bob Iger was scheduled for Good Morning America... well, nix that in an embarrassing way. McDonald's Happy Meals coordinated with opening are now many months too early.
Special flavored French fries you ordered from a food vendor for your land are now needing to be cancelled or stored frozen longer, etc. and some items are perishable.
A longer construction timeline, with buffering for each phase along the way, is the best way to make sure all this stuff comes together on that date.