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 &quot;resorting&quot; 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:
$\ce{CH_3MgI + CH_3COOH -&gt; just a salt}$
$\ce{CH_3MgI + CH_3CN -&gt;[Cu^+] (CH3)2C=N-M -&gt;[H2O] (CH3)2C=O}$