The theory was that although Kirk would recognize goggles, or protective eyewear or even sunshades---he wouldn't easily recognize the small, spindly Fraklin 'half-glassess' readily.
Of course, if he was alone he would have examined them and figured it out. But seeing as how they were a gift and McCoy was standing right there, he just says, "What is it?"
Not a mind blowing leap for such momentary confusion, IMO.
And yeah folks the ABC/Director's cut that includes the extended dialogue has been seen since 1982---you might want to check it out!
Yeah, he'd have figured it out, but the script calls Romulan ale an INSTANT DRUNK, so maybe Kirk was already a bit addled when he saw the glasses. And then, there is a knack to wearing and using reading glasses properly for the first time.
I always thought that while the glasses were a nice antique, they were probably a gag gift or given to make a point. I doubt McCoy really meant for Kirk to ever wear them. I think the glasses were McCoy's not so subtle way of broaching the subject of Kirk letting himself grow old rather than getting back into the action.
Also for what it's worth, what happens to all the other farsighted people allergic to Retinax V if no one wears glasses in the 23rd century? Do they stay farsighted? There must be other ways of correcting their vision, like a form of 23rd century laser correction surgery.