I like writing my own rules ...
... because I think that wargaming as a hobby is about designing your own game, not playing someone else's game.
Just as we hunt for the perfect figures to use, paint them ourselves and engage in building scenery, I feel that constructing your own rules is part of the wargaming experience. A wargamer should design and build the entire game himself. But of course, one should not be too dogmatic about this philosophy. After all, I don't sculpt and cast my own figures, nor do I carve my own dice out of wood.
... because published rules also contain quite a lot of arbitrary design choices.
I started out my career as a wargamer in my late teens and early
twenties, I slavishly followed published rules. My impression at the
time was that wargaming rules were the perfect representation of how
battles had happened, and that rules designers spend years honing and
polishing their rulesets, based on original research. However, after a
couple years, you start to learn that published rulesets do contain a
lot of arbitrary design decisions, ranging from game mechanics to army
lists to point values to die modifiers; and that the "deep research" is
not so deep after all. Once you realize that a published ruleset is just
written up by another fellow wargamer instead of an all-knowing demi-god
- the step towards modifying published rules or writing your own
suddenly becomes much smaller.
... because wargaming is about telling stories, not simulating war.
Although I know that some wargamers see a wargame as something that should recreate the commander's experience, I see a wargame more as a tool to tell stories inspired by military history. I understand that if you consider wargaming as a way to simulate (for lack of a better word) a real battle, a well-researched ruleset is a necessity. But I have evolved towards another approach, in which the purpose of a wargame is not to learn something about how battles were conducted, but as a way to tell stories inspired by military history. In that sense, the rules should support storytelling, and should not support the idea of running a simulation. This provides much more freedom in writing rules and injecting elements that are not aimed for historical recreation, but are aimed at providing drama at the gaming table. As such, one should not be too worried about the historical validity of wargaming rules. As long as the rules provides an exciting game, it's ok.
... because developing rules is fun.
I like tinkering with game mechanisms. I like playtesting various ideas. I like the creativity. That's why I like wargaming as a hobby so much!