Because they want to mine their platforms for more profit at the expense of their users/customers. It's just how capitalism works.

If you look back over history corporate changes like this tend to happen in waves. like for example a few years ago Disney and Warner Brothers and HBO etc. all took their programmes off Netflix and created their own platforms at around the same time. This is because they are all copying each other. One corporation thinks up a new way to harvest money from their consumers and all of that corporation's competitors do the same because they don't want to miss out on all that cash.

Reddit is pushing their third party apps out of business because they want to force their users onto only the official reddit app. Probably they also have a plan to further monetise their app in the near future as well (e.g. way more ads or paid subscriptions tiers that unlock more features). Youtube closed down all the third-party apps because people were using them to get around the ads, and YouTube wanted to force more and more ads and also the subscription service because that's where they get their money from.

Literally the only reason they do it is to increase profits.