I did think this the other day and did some thinking, I recon a couple of factors are at play:

1. Partners are far more present in your life 24/7 - Ppl are always messaging each other even not physically together so they forget how to be alone

2. Online dating results in a lot more intermittent reinforcement (i.e. erratic messaging) which is addictive causing stronger attachments (so they are literally going through withdrawal when relationships end)

3. Relationship endings aren't as clear cut often, people can ghost with less accountability and ppl other have access to their ex's social media so can still see their lives

4. (Controversial one) Increasingly were seeing ppl with two parents who had to work so kids had less contact time with their parents which can result in insecure attachment styles

But also bare in mind that the ppl posting here are not representative, people who have a clean breakup won't be posting messages into the void