I think God knew what they might be tempted to do if they could keep portions of any offering: they’d become butchers and chefs. They’d get the best meat, get good at seasoning and grilling, maybe they’d sell some portions, etc.
Eli’s sons later almost got to that point, but they were too lazy.
We always need to follow God’s instructions even when we think we know better practically. He invented practicality, but He also invented provision. Trusting God has proven more “profitable” than going contrary to His will.