There's a popular idea that if you have a need for something then it's likely that other people will too. It's often termed "scratching your own itch".

Developers like to take this idea and assume that if there's something they need or want and build an app to meet that scenario others will also buy that app to meet the same need.

I think this view ignores a big issue.
The "others" in this scenario are typically other developers, and developers like writing code to solve their own problems.
The corollary of this is that developers don't look to buy solutions written by other developers to solve their problems.

Yes this is a huge generalisation but I believe it also holds a lot of truth. Just ask a developer you know how often they buy solutions to their problems verses writing their own solutions.

The mis-assumption is that other developers will buy their app as they have the same problem because they are just like them. The problem is that just like them the other developers will be creating their own apps to solve the same problem.

Developers building small apps for other developers isn't a great way to find a big market and sell lots of apps.
By all means build such apps just don't get upset if you don't get millions of downloads.