Aside from not knowing the 49ers' pick, much of the staff thought it'd be Mac Jones. Shanahan let the staff marinate in smokescreen:

"When you get an opportunity to go through something like that, it is really cool to find out about people in your building, how they handle it."

I theorize Shanahan did this to ensure he'd get the most honest work possible out of his scouts/staff in pre-draft research process. Divulging Trey Lance was the favorite might've led to groupthink. Letting the Mac Jones smokescreen live led to more healthy debate, pushback, work

Because if the personnel department naturally favored Trey Lance to begin, simply broadcasting Trey Lance as the favorite could’ve led to some resting on laurels.

Plus, that would’ve opened the 49ers up to outside leaks. And they didn’t want to risk tipping off the Jets’ staff