Because "atheistic" refers to the absence of a thing, while "theistic" refers to the presence of a thing.
The only thing you know about "atheistic ideology", is what the ideology does NOT include. You don't know anything else.
While a "theistic ideology" literally refers to the presence of very specific things.
You KNOW it centers around a belief in a personal god. As such, you can have a basic idea concerning what it is about.
Both give you information.
Given the difference between theistic belief systems throughout history is about as great as those between atheistic belief systems, it gives you about the same information, or close enough not to quibble it.
There is also debate if it necessarily refers to a personal god as you can see in this thread.
This is just false.
Theism as a word tells you a whole lot about what a person believes.
Atheism only tells you about what a person does NOT believe.
One says you believe in god, one says you don't.
Both are equally useful information.
If someone tells you they are an atheist, you know something about them and can often make assumptions with reasonable confidence based on the circumstances. Same if someone tells you they are a theist. No information exists in a vaccum.
If told "Person X was an atheist from Germany/Germania and Person Y was a theist from Germany/Germania", you would probably be better able to predict characteristics of person X imo (obviously with a significant margin of error).
All you know factually is 1 thing either way though.
True. But beliefs inform actions and decisions. Disbeliefs don't.
This really shouldn't be difficult to understand: disbeliefs inform actions and decisions too. Disbelief is simply adopting the position that X is not true.
You honestly don't think you are impacted in any way by things you decide are not true? As someone who cares about reason and rationality, you can't seriously be saying that surely?
My atheism certainly influences how I view theistic belief systems. My disbelief in ghosts influences how I view bumps in the night and 'faces' in the curtains. My disbelief in the healing properties of prayer and homeopathic remedies influences my response to illness, etc.