This (understandably) comes up often and it would be a huge quality-of-life update to fix this across the game. Unfortunately it's not super simple, but the reasons that OSRS is the way it is are quite interesting.

The original philosophy of RuneScape's engine and scripting language was "write once, run everywhere", maybe taking some inspiration from Java. In the context of RS though, this means that a developer would write their game scripts once, and not need to worry about intricacies of a networked game (like whether Player A's client showed something different to Player B's client, or latency issues if server-side scripts need to hand over to client-side scripts). The earliest versions of RuneScape 2 had virtually zero gameplay code running on your game client; it would simply receive and display updates from the game server, and send inputs back. This was true not just for in-world gameplay, but also for menus and interfaces.

So, all game scripts originally ran on the server, which controlled both gameplay and interfaces. The scripting language was also designed so that you wouldn't need much technical ability to write a quest or make a fun diversion, and so it's FULL of safety nets to stop developers accidentally breaking things. One of these is that your character can only be doing one thing at a time, including looking at an interface. When you open a skill guide, that interface must forcibly take full control of your character by stopping you from fishing, because there's no way for the engine to know whether the open-skill-guide script might have some funky side-effect that would break the fishing script. Similarly, you can't move while an interface is open in case a leave-an-area script would break the interface (this is why you sometimes stop running when the fletching Make-X menu opens).

For a long time this was the only way to make an interface, and remains one of the easiest. We have newer tech now that allows much more responsive interfaces that are allowed to stay open (the Activity Advisor and World Map are good examples), but because the old way was the only way for so long, there are TONS of interfaces across the game that still have this restriction. They'd each need pretty hefty rewrites to remove this annoyance, but (and at the risk of committing us to doing it) that's not to say we can't do it for a handful of heavily-used interfaces.

tl;dr: old interfaces are old and there's a lot of them and I love this silly old game