It's a few clicks in any browsers dev tools or a few more clicks in the settings, and is the same for every website.

With a native app the cache could be in any number of places on your hard drive, you have to depend on a function being in the app to clear it, deleting it manually might have unknown side effects, and it's all nonstandard.

Websites can’t write to arbitrary places on your FS and the browser has a central way to delete the data that a website can write.

Desktop apps can write anywhere, and what it writes might or might not be a cache, so there’s no way to centrally manage it. Apps like cache-cleaners simply hard-code common cache paths for common apps.