0. It's not like the vaccines get discarded or the factory slows down - every dose that isn't purchased by rich western countries is one that will go elsewhere, possibly saving even more lives.
1. Professionals might have data we don't have - maybe they've used up their stock of AZ but have plenty of other vaccines.
2. They might be making more nuanced calculations. For example, if they've already vaccinated all their high-risk groups, the risk-to-reward trade off for younger groups might be worse.
3. Alternately, they might be being more conservative than you and I. I'm expecting the end of vaccine roll-outs to mean lockdowns and masks end permanently and the entire economy comes out of hibernation, which is a big pay-off. But between the vaccine's less-than-100% success rate, the risk of new variants, people who can't be vaccinated, international travel and so on, they might feel my benefits are too speculative to have a place in their calculations.
4. Healthcare ethics can be a complicated matter, and they've probably spent longer agonising about it than we have. After all, you and I are talking about pulling a hypothetical lever - they're the ones who'll actually be pulling it and cleaning up the splattered blood.
5. They might be particularly worried about the vaccine-hesitant, feeling hesitancy is a bigger risk than the deaths caused by vaccine rollouts taking a few months longer. They might feel the best way to reassure the vaccine-hesitant is to be demonstrably extra-vigilant, rather than providing reassurance that might sound like denial or cover-up.
6. Public health comms has to be inclusive of even the dumbest people in the population. Nuanced, complex messages might get lost in the mix. With this, the message is "WE NOT GIVE YOU BAD VACCINE. YOU GET VACCINE WE GIVE YOU GOOD VACCINE." - very simple!