Traditionally, books were sold to the relatively educated relatively literate middle classes.
Kindle opened up an area to the left of the bell curve, pulled in much less educated and much less literate readers, and created a new class of much less educated (etc) writers producing content for them.
These new readers are much less picky about essentials like grammar and continuity. There are also far more of them. Numerically, the literate+educated market is much smaller than the barely-graduated-high-school market.
There's also a big teen lit audience which hasn't learned enough about grammar or fiction to be horrified by writing that looks just plain incompetent to educated etc readers.
Publishing rediscovers these markets every few generations. Previous iterations were pulps, potboilers, and penny dreadfuls.
There's a solid core of wannabe writers of all ages with much lower literacy than average. It's a bizarre thing. Their FB and forum posts have the same mistakes their writing does, and their writing is full of the usual potboiler cliches - murder, torture, sex, violence, maybe some magic, lots of shouting and running around.
Some of them are doing okay commercially, so that's just where are now.