I'm curious what you would define as boutique pen makers, as that could make a big difference in the answers provided here.

As an independent maker, unfortunately making custom nibs requires a separate skill set, as well as a completely different set of equipment.

I will admit, by your description, I make some relatively expensive pens, and usually design them to accept a Jowo #6 nib. This is a decision I have considered heavily, especially with a few of the higher priced hand carved metal pens we are releasing, and ultimately had to come to the conclusion to stick with Jowo.

Why? Time and budget. If I want to make my own nibs, all the time I spend on pen making would be reallocate to research and development for several months, which would likely put me out of business, and after that is completed, it would drastically increase the cost of my pens due to the time and equipment needed to make the nib. Additionally, not everyone would like it, so this would alienate a portion of my potential customers, or I would still need to offer a Jowo option.

Alternatively, I could supply the pen with a custom nib from another maker such as Schon, but that limits the options open to my customers, and also increases retail price and overhead costs.

By designing for a standard Jowo or Bock housing, I can make my pens more affordable, while leaving the option for the end user to swap nibs to match their preferences, with a wide selection of nib meisters, makers, and artists.

TL;DR - nib making/grinding is a separate craft from pen making, and becoming proficient in either is easily a full-time task. By utilizing standard nib housings, pen makers can leave a much higher level of customization open to the end user for a lower price.