I can't generalize to everyone's motivation but I can tell you ours when it comes to Clarity.
Iconography is an important piece of a design system. Designing iconography that fits within the general look and feel of a design system ends up defining a big part of it especially if that design system is used across many application (within and outside of VMware) as Clarity is being used.
With that in mind, we ended up designing our own. Since Clarity itself is fully open source under the MIT license, it only made sense to open source the iconography system itself and the icon set.
Icon fonts don't degrade gracefully when someone (ie me) disables "let websites use their own fonts". Either you get meaningless letters or those little unicode squares - neither of which are actually useful, of course.
Additionally, icon fonts rely on the PUA to render their glyphs. Given the emoji explosion over the past several years, that PUA is getting harder and harder to assign glyphs to.
I can't be the only one that's visited a site on my phone and found emojis where an icon font glyph was meant to be.
The icons look blurry on most zoom levels. On zoom 90% the effect is reversed and the left icon looks ok but the others don't: https://imgur.com/a/AeREO
Iconography is an important piece of a design system. Designing iconography that fits within the general look and feel of a design system ends up defining a big part of it especially if that design system is used across many application (within and outside of VMware) as Clarity is being used.
With that in mind, we ended up designing our own. Since Clarity itself is fully open source under the MIT license, it only made sense to open source the iconography system itself and the icon set.
If you take a look at the link posted, there is also an interesting iconography system behind this (You can read more here: https://medium.com/claritydesignsystem/the-road-to-svg-and-c...).
Does not answer your question in general but thought I'll offer our own point of view.