Chemistry - Why can't the reaction of grignard reagent with carboxylic acid give a ketone?

Huy Ngo says it all. A Grignard reageant will react first with just about any hydrogen that has even a modicum of proton donation capability, before "resorting" to the slower nucleophilic attack.

To get a ketone (in most cases), avoid the dissociable proton by using a nitrile instead, assisted by a copper salt. This forms an intermediate imine which is hydrolysed to a ketone after water/acid workup:

$\ce{CH_3MgI + CH_3COOH -> just a salt}$

$\ce{CH_3MgI + CH_3CN ->[Cu^+] (CH3)2C=N-M ->[H2O] (CH3)2C=O}$