For the same reason that other recipe specifics aren’t stated in the guidelines. For the beer drinker, its all about where the beer ended up, not how it got there.
I suppose one could argue that pH identifies a characteristic of the beer the same as the OG, FG, ABV, IBU, SRM and ingredients list. Are these characteristics also not used by brewers?
The BJCP Guidelines are not a recipe guide. They are guidelines for judges at competition.
If someone feels strongly the guidelines should be revised in any way, they should write up those changes/styles in detail and send them to the Style Committee.
To be clear what I mean by that, if you were to send an email to the style committee and say, “please add mash pH to all the styles in the BJCP Guidelines” or “the International styles are wrong”, your email would probably get ignored.
The email should be style by style, here are the changes to that style. There are Word documents on the BJCP website where you could track changes.
Aren’t these guidelines where most brewers go to get parameters for the beers they brew such that they fit into a specific category? Or do most brewers just make a beer and then decide which category to place it in?
There are many unspecified parameters. Mash type, mash temperature, mash thickness, fermentation temperature, pitch rate, oxygenation levels, and so on. Those are for the Brewer to figure out for the beer on her/his system with the process used. pH is just another one that the Brewer has to figure out.
The guidelines describe the finished beer, not the process to get there. The pH, indeed the entire recipe, can be whatever you want as long as the finished beer fits the guideline.
I guess a more advanced extract brewer could, but that takes the simplicity out of extract brewing. IMO simplicity is the main benefit to extract brewing.