Hi, Can Anyone Explain Why People Often Say You'll be "Locked into Amazon Ecosystem" if You Buy Kindle, but no Such Thing for Other Brands? Aren't They also Have Their Own Bookstore/Ecosystem?

I personally have a Kindle, but apart from a handful of cheap books I bought directly from Amazon, 98% of reading materials in my library are fanfictions and books I obtained by other methods. All were sideloaded using Calibre & sendtokindle website (sometimes)

Thanks in advance