That's intentional. Separate apps mean easier development, maintenance, and deployment. If everything was on a single app, code changes could break existing functionality in other parts of the app requiring a ton of regression testing before release. As separate individual apps, Nintendo can roll out bug fixes and improvements much faster.
TL;DR: You should want separate apps. If one app fails, it won’t take the other functionality down with it, and it's much easier to fix.
*shrugs*
I get what you're saying but my home screen has room for 1 app, and I prefer the convenience of doing everything I need to do in one app. It's frankly presumptuous for Nintendo to think it has a right to more than that space. Just saying, the PS App has its issues, but I can buy from PSN; manage my downloads; managed my screenshots and videos; and manage my online profile all in one place. Hell, when PS Stars was still active I could manage that there, too, and so long as PSN itself is operational it works just fine and has for years.