They’re also just more user friendly in general. I have a nearly decade old Kobo that still serves its purpose well and I love how it shows up as a mass storage device when plugged into my computer. No clunky management software (Calibre may be functional, but its UI is godawful) or file conversions required, you just manage plain old epubs in your file browser. That functionality can’t randomly vanish one day, either (unlike e.g. Amazon’s email-to-Kindle service). It’s great.
As a side effect of its openness I’ve bought more books through the Kobo store than I would have otherwise. A perfect counter example is my Oculus Quest 2 which I go out of my way to not buy games from its associated marketplace on; as long as that device is hard-tethered to Facebook and its software, I’m better off as using it as a “dumb” headset with VR purchases going to PC marketplaces like Steam.