Belief that hobbies must be difficult to be enjoyable is a matter of personal preference. Hobby is doing what we like. Hobbies can be different: singing songs, playing the guitar, running in the morning, watching movies, etc. We get pleasure when we do something like that.
It’s hard for me to answer whether I agree or disagree. Some hobbies are easy like watch a video, play the game, hanging with friends. When we do something like that, we don’t develop, but it doesn’t mean that we shouldn’t do it at all. Everything needs moderation.
When we do something, we like, we don’t know how to do it. For example, if we want to play the piano, first, we need to learn sheet music; second, we have to learn the whole composition by heart; third, we need to use piano technique, etc. It is so difficult because we spend a minimum of three hours a day. Every small achievement provides us with incredible satisfaction and a sense of accomplishment. When we talk about when people strive for something, we can agree that hobbies must be difficult in order to be enjoyable.
However, hobbies like look at the sea, walk in a park, travelling give us more enjoyable. It depends on many factors: age, gender, how we relax, etc.
In conclusion, I agree some people believe that hobbies need to be difficult to be enjoyable. We have different interests and preferences.