Because it makes them money. See, monolithic companies don't care about anything other than money and pleasing shareholders. The only way they will ever care about real life issues is if/when they transform into loss of revenue or pressure from their board of directors.
Shareholders can be batshitcrazy too. X/Twitter has been in decline for a while. It's true they could be leeching money from the he far right but I highly doubt so.
Yeah, private companies at least have the opportunity to forego greed to ensure a better product.
Heck, even some public companies are able to do it if they can prove to shareholders going the greedy route is a bad business decision (an investor floated the idea around Nintendo that they should charge money to make Mario jump higher.)
TBH it's not just reddit or social media: Internet's per-view ad revenue is exactly what ruined traditional newspapers (ok, some where shit to begin with)