I think a lot of people that feel that gaining muscle is easier are perhaps deceiving themselves with regards to their actual gains.
Some people can put on say 3 or 4 pounds of muscle and 5 or 6 pounds of fat at the same time and still look very lean in doing it. It all depends on how you store fat, how tall you are, what type of frame you have. Heck, even how tan you are has an impact on the visual appearance.
I would be wary to believe someone that says they can put on muscle easier than they can lose fat without knowing exactly how they have determined just how much muscle:fat they have been gaining.
I think it is pretty easy to gain 10-20 pounds in a relatively short amount of time. Probably about as easy as it is to lose 10-20 pounds. But, the 10-20 pounds you gain is unlikely to be all muscle, even if you don’t look like you got much fatter in the process.
Overall, I think if you’re overweight, getting down into a more normal weight is going to be a lot easier than if you are underweight and trying to get into a more normal weight. Unless you completely disregard muscle and just eat to get fat, in which case I think both goals would be similar.
I know personally I have to gain at least 10 pounds to even look like I have changed at all. I’m 6’4 and I have a naturally big frame, but I have been very skinny my whole life, now I am more normal weight, and it has been a challenge, I probably gained more fat than I would have liked in the process, but you would barely be able to tell because I am so tall.
Likewise, some friends of mine are shorter and they can gain or lose a couple pounds and it is immediately noticeable, but they don’t necessarily gain or lose any faster than I do.
I do think that in general, people put far too much emphasis on losing fat than on gaining muscle. It seems like the majority of readers on this site are overweight trying to get trim versus underweight trying to get big. Even if that isn’t necessarily the case here, many of the articles seem geared towards that.
Which is not to say skinny guys are left out, just that there is a slight bias towards fatsos trying to lose weight, and I think that is unanimous across the fitness industry, and I think that is the case mostly because it is easier to do in less time, so it is easier to market and sell.