The term "Middleware developer" is not what people talk about. They talk about "game development", which includes middleware development, they are not two separate, exclusive fields anymore.
Your missing my point, those two things go hand in hand and as I already stated, is much more valuable to be well versed in both, more so the latter because it is generally a more difficult technical skill to acquire that talks years of discipline.
As you pointed out earlier, this is a forum for people wishing to break in.
In order to write an effective reusable component (be it a key library, a tool, or an engine component), you need to be extensively familiar with the requirements. That is a level of seniority and experience which doesn't apply to someone entry level.
They are far better served by spending their time *using* an existing and solid framework to make actual games. If they ever do need to write their own engine (or be a key engineer writing an engine for an employer), they are much better served by having made games than they are by making toy engines.
I'm just saying, in a day and age where everyone creates a home-brewed engine of some sort and learns the knowledge required to do so, is much more desirable than familiarity with click/drag operations.
What you need to do to be a game developer is make games.
Back in the late 1990s, there was a game development channel on EFNet. Mostly aspiring game developers, and a handful of professionals.
And they would say "I am adding this to my engine" or "I am going to redo my engine to support that", and we would say, "Just make a game."
And they would talk more about the technology they would add to their engine.
A few years later, I wandered back in. Same people, mostly. Still talking about what they would add to their engine, still never having actually made a game.
If you want to be a game developer, make games.
If you want to be a middleware developer, make middleware.
Like Promit already pointed out, what engine you use does not matter. But here is my opinion, using tools does not make you a game developer. Anybody can pick up a set of tools and create something passable because somebody else has done all the hard work in creating those tools. Be that guy if you want a job and to be successful.
By implication, you would need to therefore write your own compiler, and OS, after first designing the CPU you intend to use.