Why are clearing turns supposed to be shallow bank?
Clearing turns solve two problems: seeing other aircraft and helping them to see you.
Banking will make you more visible to other aircraft, who may not have noticed you before. A lower bank angle (and thus turn rate) gives them more time to see and avoid you. But a very low bank angle also doesn’t make you as visible in the first place, so it’s a balance.
I was taught 20-30°, and I’ve never had anyone say later that was too much or too little. I also don’t have to worry much about losing altitude at moderate bank, so I can keep my eyes outside scanning for traffic—not checking the altimeter every few seconds like during “real” maneuvers.