In general it’s because it’s difficult or impossible to fully monetize a user on a third party app. This could be because the API doesn’t serve ads (like Reddit) or because it’s harder to track and harvest user data when they are not using a first party app.
Essentially, platforms like Reddit make deals with advertisers that they will display an ad in a certain format near certain types of connect (and away from other kinds) and will show them to users with specific interests. They can’t really do that if the user isn’t coming through a platform the company fully controls (so their website or apps). On top of this, their apps are designed to keep you engaged as long as possible and to harvest as much information about you as they can without you getting upset and leaving. This lets them target ads more specifically (which means they can charge more for them) as well as sell that data to brokers for even more money (who then sell it to advertisers). It’s all about how to best turn your attention into money, and a third party app doesn’t allow that (either at all or as much as a first party experience).
Reddit specifically also wants to sell access to their data to companies like OpenAI to train large language models as additional revenue sources, to do that they need to lock down the API used by apps to work with the platform.