Allow comments checkbox not checked for 2k+ posts

Whenever you change the default allow comments setting, it only affects new posts.

If you have access to your database via phpMyAdmin, you can change all the posts at once using SQL:

update wp_posts 
set comment_status = "open"
where comment_status = "closed";

Change “wp_posts” to match your posts table.

Or, you can do it using the Bulk Edit method on the Posts page. This thread talks about it: https://wordpress.org/ideas/topic/turn-comments-onoff-for-multiple-posts-at-once