could be many reasons, really...
- the content of those articles is garbage
- 100% AI content
- crawl budget (google are having limited resources nowadays, and if they've already used them all up on other pages of your site you might need to wait until next month to get google to crawl - and index - these pages)
- orphaned pages (pages that have no links and no traffic coming into them are considered orphaned, therefore not valuable, therefore not worth indexing. Just throw some links - both internal and external - at those pages and re-submit sitemap to GSC... or request manual indexing of those links, although if they're full of AI / garbage content requesting indexing won't help much because they might get de-indexed days later... or wait for google to discover them on their own)
- time (google are very slow at indexing stuff nowadays, especially stuff that they don't consider important / quality, so those pages might never get indexed)
- technical issues (did you check whether those pages are not noindex'd? Because if they are noindex'd... well... good luck indexing them

)
If the pages are not noindex'd you can share them on social media and other UGC sites (quora, reddit, imgur, etc) and / or throw them into a paid indexer and if they still don't index even after this much effort they're probably low quality and not worth google's attention