Can’t speak for others but the bizarre Android fork Fires run is a big part of what makes me rule them out.
That, and I have an Android tablet that was ~$250 when new that has various aspects about it that are somewhere between awful and underwhelming, and I can’t imagine a $30 Fire tablet to be any better in that regard, subsidized or not. That said my tolerance for electronics being bad in any capacity I feel on a day to day basis is low.
I've had a lot of tablets. iPad is better than most of them but my feeling is that tablets are sorta disposable, even if you don't intended them to be disposable. [1] If you want to read the web, read PDFs, it doesn't really matter that you're running AMZN's fork and using AMZN's gimped app store because you need like: a web browser, a PDF viewer, maybe ssh and RDP clients and that's it.
The only (relatively) high end Android device I had was a Google Pixel, everything else has been a medium Samsung, BN Nook, I even had a $99 Windows tablet that was "good enough".
A lot of people who would want a Kindle would be pretty happy with a Fire, it is not so easy to read in peak daylight and the battery life is worse, but the value is good enough that AMZN has driven quality affordable Android tablets out with very little fanfare.
[1] I can't believe anyone gets an iPad Pro, but if I did I'd get an expensive case for it and treat it like a laptop. What I like about tablets is that I can take them places where they might get smashed.
Even if they’re technically sufficient in terms of specs and my use case is simple, if I see things like animations hitching, times when the tablet is struggling (e.g. while running updates), the screen being sub-par (low pixel density, bad viewing angles, etc), etc it’s going to bug me.
On tablets being disposable, it depends on the user. The only electronic device of mine that’s regularly in harm’s way is my phone, and it’s insured so if it gets smashed I have a new one in a day or two. Been carrying iPads as auxiliary devices while traveling for years and never had a problem.