Because "mechanic" is an occupation.
We don't talk about "fishermaning".
There are many things we don't say, that by following the rules of English, we could say, McTag.
"doctor" is an occupation, as is "nurse" and "engineer"
Exact phrase Google "mechanicing"
About 5,360 results
That's certainly respectable for a relatively new addition to the language.
But we're getting away from the original question. For NaE, it is quite common. How it fits, actually fits, in BrE is another question.