It’s not just Reddit, so many companies try and shunt you off a mobile web page and on to their app, despite many apps being little more than a pre loaded mobile web pages.
Why? Because users can modify how they interact with a web page, they can install extensions that modify how the code from the website is run, or just deny web pages access to some other process. There is very little a company can do about that, they have no control on how the user chooses to run the page. But… with an app, users can’t modify how the program is run. No plug ins, no web extensions, no choosing not to run some part of it, just the software as distributed by the company. Meaning full fat ads and complete access to any information the OS will let them have, way easier to make money on users that way.
Technically, it’s possible to alter any program, but it’s very hard if don’t have the source code, and it’s illegal to do so in many cases thanks to section 1201 of the DMCA, especially if you try and distribute that modification or tell others how to do it. Which is dumb, it’s your computer/phone, they shouldn’t get to tell you what code you can and can not run on it, they shouldn’t be able to force you to run code on it you don’t want to.