What really is inflation?
This is a really big topic and you're essentially questioning why don't we calculate price levels using Paasche's indexes instead of Laspeyre's. Well, one "simple" answer is that's it's very expensive to maintain a detailed account of what a representative consumer basket should be. 

There's people trying alternatives to this, like the Billion Prices Project, but their estimated inflation is very close to the one we estimate through traditional, constant consumer basket method.