Because, why would they in the first place? How would blockchain technology help Facebook and Twitter's platform exactly?
Remove your mindset of that every single thing should have it's own blockchain. Blockchain is very unnecessary in most platforms, hence why we see a lot of useless coins and tokens.
You are right that there are lots meaningless coins and tokens, they do not even solve real problem, and some existing products are much better than them.
But just wonder what blockchain will bring to facebook, twitter these social media.
The advantages and disadvantages, and how they evaluate.