Because the CE buff probably wouldn't be as much as you think.
Gege has been pretty persistent with clowning simple one-time set ups.
Miwa didn't get even get enough CE to bypass Kenjaku's reinforcement.
Kashimo.... well you know lol
I think Gege's mockery of one time power ups is intentional since HxH made Gon's adult mode too OP and it created the same basic issue.
I think a good explanation is that the strength of the binding vow depends on how big of a sacrifice the person thinks they’re making. If someone knows they’re about to be killed and they’re hopelessly outclassed, trying to make an Adult Gon type vow in that moment wouldn’t yield much of a boost since the user really isn’t making much of a sacrifice in that situation. However, if they made the vow prior to a fight which they thought they had a decent shot of winning regardless, they might get a much larger boost out of it.
I wonder if the “revealing one’s hands” binding vow works under the same logic. If you have a very straightforward CT like element manipulation, you’re not sacrificing much by explaining your CT to your enemy. However, if you have a tricky CT which you gain a lot out of by not having your opponents know how it works, like the inverse damage CT, you are sacrificing a huge advantage by explaining your CT, so you would deserve to get a greater CE boost from explaining it