Fun fact! They weren't always prefixed. And then after someone spent hours pouring over the logs in Splunk, that's when they had the idea of prefixing then.
I assume it's loose convention intended to identify the string as "Secret Key", as a hint to the humans who might be moving the data around, and maybe to make it easier to write precautionary code like "hey it looks like your commit has a secret key in it, are you sure you want to push it?"