Spark Plug Blow-by

I just had my car (2016 Mazda 3) looked at by a mechanic who recommended replacing the spark plugs because there were signs of what they called “blow-by.” I had never heard of this before so I looked it up and I find it weird that they only wanted to replace the spark plugs. There was no mention of diagnosing the cause of the blow-by as the spark plugs themselves don’t seem to do that on their own.

Are they trying to scam me? Is there a reason it would only be the spark plugs causing the issue? It seems like a lot of other factors could be at play.