Ghast fireballs are targetable and can be configured to display any desired item. While one might anticipate obtaining the displayed item when using the pick block function, this is not currently the case. This relates to MC-269941 and should also remain open for Mojang to determine whether this behavior is a bug, intentional, or a feature request.

Steps to reproduce

1. /summon minecraft:fireball ~ ~ ~ {Item:{id:"minecraft:snowball"}}

2. Try to use the pick block function on the fireball

Observation: No action occurs.
Expectation: You should receive a snowball.