I think that's just lightening are drawn usually. As You can check Don sai , Who uses some martial arts also have Lightening like Sanji Ifrit Jambe.
That's the neat part. We don't have explanation.
WB never amped his attacks. He only used Haki as a medium to hit Logia types. That's how haki works.
The way I see it , Haki hard counters the user themselves.
- Katakuri If coats his Punch in Haki , That's a solid material now though have Mochi property but A full bodied armananet Katakuri can Never shape shift like Logia types.
- Kuzan Ice Punch , makes his hand solid. I can bet None haki users can Attack that specific part too.
Similarly for Law ,
He has been given alternatives to Haki ,
Internal Destruction - Injection Shot.
Shockwille - AdCoC type of AP.
Puncture Wille, - AdCoC type + AdCoA. + AoE.
Now if Law were coat his blade in Haki , That's a solid material now , If Law uses K-Room on top of it. How does his sword even Pass Through things?