As a forty year user of fountain pens, I must thoroughly agree with you. Moreover, most modern pens, regardless of the brand or price, lack the panache of those made before c. 1960. To the point, however- modern nibs are a pale imitation of the 14k pre-1960 (or so) nibs, and the nib is where the rubber meets the road.
A further issue with modern pens is that most either use cartridges or a converter. Why have a pen that the palette is limited by the few choices of colours offered by the manufacturer's cartridges? It seems to defeat the whole purpose. Cartridges and converters have a small ink capacity; the danger of running out in the course of the day is very real. An older piston-filler or better yet, eye-dropper pen, will write for days and days before refilling.
An answer to why more than one pen of the same brand: different colours of ink. I kno those of you raised in the ball-point/Bic era won't understand that... when on business I carry two or more pens, one sedate pen, gold nib, conservative blue, black or blue-black ink therein and another pen, more flamboyant, containing a more brilliant colour (which is used to mark up documents).
All of which begs the real reason to write (not just carry, ahem) with a fountain pen - the shear pleasure of the experience.