and it's because those exotic powers are in Mjolnir.
All these years we'd have seen him doing them w/o Mjolnir if he was capable of doing them without Mjolnir. But he NEVER does.
Plus, w/o Mjolnir, he can't power up. Bill couldn't power up during the invasion of Asgard by the Skrulls until Thor tossed Mjolnir at him. If it was Thor's power, if it was Bill's power, then it would be in them and they wouldn't need to power up, they'd just summon it all on their own. But they can't... unless they have Mjolnir/Stormbreaker.
Many of the exotic powers ARE in Mjolnir.
It's also the reason why we NEVER see Bill doing them w/o Stormbreaker, either.
The power may be theirs to command due to their worthiness, but they cannot get to it on their own, they MUST have Mjolnir/Stormbreaker in hand to get at it.
Wait what "power up" are you referring to with Thor? There are examples of Thor using both weather or his Godforce without using mjolnir.
As for BRB in Secret invasion, keep in mind WHERE BRB was BEFORE the Skrulls had him. He'd been in another dimension keeping a demon from entering our dimension.
The Skrulls had taken Stormbreaker to experiment on as well as experimenting on Bill, so of course Bill's going to be changed to his mortal form. (which doesn't make any sense since Bill's mortal form is a creation from enchantments applied by Odin and his Cyborg body was destroyed in the mini following the Ragnarok arc and they seem to be ignoring Simon Williams completely.)