as said by Josh Perrie above:
"Not built here syndrome;"
I am also writing my own engine, and I suppose the reason will be different for every developer out there, but in fact - I generally don't like working in other peoples code. I am compulsive in the sense that If I feel I could build it myself, then there's no point in settling for anything else.
I tested out various types of game engines, rendering API and such, notably Ploobs, UNITY WaveEngine, XNAFinalEngine, Love, Ogre, etc.. many more... I wanted to start writing games - I downloaded a lot looking for a good comfortable and well documented entry point...
My problem however, was at the time I had no idea what was happening below the engine. I wanted good control, and I wanted a framework I know like the back of my hand. I came up with the idea "HEY! I think the only way I'm gonna learn how the thing works and understand it is to try building my own engine entirely and completely from scratch. Most of my programming history was with web and processing solutions - this was a whole new ball game for me.
Which is what I ended up doing.
So I chose to set up XNA since I already knew C#, and started thinking about how or where I should start. I needed an idea.
I decided that, no matter what, I would go straight into 3D.
Getting the basics down was cool - the sprite batch stuff, but as I progressed I ended up discovering new barriers and obstacles - my first real one being the batch limit. My goal was to build a game that could at render at least 10000 entities in the view frustum at any time.
I embarked on a new journey of implementing Shader Based Instancing (and learned HLSL while I was at it) , I ditched XNA's built in Model and Effect objects to write my own replacements instead. I had trouble understanding the VBO streams at first; I broke things - I went online asking questions about the instancing stuff and kept at it until I finally understood what the GPU was doing. It paid off; now I had over twenty thousand test entities zooming around in my viewport after a couple days of debugging my VBO with PIX (dxsdk).
Now I had "some" idea of how rendering pipelines worked, but wasn't done yet - I ended up creating my own game-state, camera, post effects, and entity objects, moved away from the XNA Content Pipeline by building my own loaders (personal dislike towards the XNB thing ), created a complicated depth sorted and blend-state separated geometry chain and also had instanced sprites and text all being projected into the game scene.
I kept adding on, fixing, changing and experimenting with this continuously for almost an entire year. In the end, it came out pretty good. I now had an understanding of what is going on under the hood, because I created it - my baby.
Now my engine was mostly stable and just about finished. It isn't perfect: the scripting is honky and the GUI wasn't great at all. But I still loved it. Thousands of lines of code, assets and media - holed up in a private 2GB git repository, and all the headaches I had to go through trying to do a type of development I never did before. Every obstacle I overcame was a lesson learned - and a relief.
I pulled off almost everything I wanted in it.
But in the end - I decided it was time to put her down.
As much as I satisfied myself writing such a huge engine by myself, with advice from the net, and other gamedev buddies, I decided that I'm gonna do it all over again - and do it better - because now this time I mostly know what I'm doing.
That project still sits tucked away in my GIT repo.
My second pass at writing a new engine (this time on MonoGame), is progressing well. When something breaks, it's easier to fix. Less mess. I hope to publicly show off my game sometime this year, because I tend to be a bit 'too' attached to my code.
In the end writing my own engine is how I learned 'how' to do it, while being able to say that I know and understand exactly what every component does, and how they are supposed to work. I actually HATE reading other people's code, especially for large undocumented projects. I want everything I use to be built by me.
This is just me though. I doubt I'll ever use a pre-made engine, probably because I think it's just more fun for me to write my own frameworks than to sit and deal with someone else's code - full control.