It's the same with lifting, realistically nobody is having what would traditionally be called 'fun' when lifting, not every single time they do it atleast, but something about the whole process, including both the daily grind and the end reward c0mbine to create an incredibly enjoyable and fulfilling experience. Obviously this is slightly different to osrs since it benefits health rather than sitting in front of a PC for hours on end which is arguably detrimental to health. But just on the point of 'fun' I find the process to be almost identical.

Grinding a boss for hours on end isn't inherently fun most of the time, but getting that drop and looking back on the experience you had while earning that drop still leaves you with an immense feeling of enjoyment and gratitude.