Because your data is teeny tiny, not performance critical, and `json.load` and `json.dump` is easy and usually less brittle then you're language's marshalling library.
Why save 10 minutes on setting up a proper database? Data is never teeny tiny, data is never not performance critical :) These decisions only bite you in the ass.
All my side projects have teeny tiny amounts of data. JSON file in the order of KB. Performance is great.
I have one project with a dozen users that handles around 50 reads per second and maybe 30 writes per day, no issues at all. Setting up a DB for that would just bite the next dev in the behind since it would need a lot more maintenance then an apache server with php.
Setting up a database takes way longer than 10 minutes. Data can be teeny tiny. Data can not be performance critical. You literally know nothing about the commenter’s application.