I just don't understand how some uncontrolled respectless fighting like MMA can be called an own martial art
A few fallacies in that sentence:
1) MMA fighting is not uncontrolled
2) Neither is it "respectless". There are certainly individual competitors who are disrespectful, but on average I see as much respect from MMA practitioners as from any other style of martial arts. To be sure, there's a lot less
formality in MMA, but formalities are not the same as respect.
3) In any case, like it or not, respect is not required as part of the definition of a martial art. It's certainly a good thing for martial artists (and others) to have and many martial arts instructors and organizations claim it as an important ideal, but it's not the thing that makes a martial art a martial art.
there are many different martial arts in it, only because you mix something up, does not mean you create a complete new and own system
Very true. On the other hand, mixing up elements from prior systems doesn't mean you
haven't created a new system either. It comes down to what you've done with those elements - have you created a cohesive set of principles, training methods, and techniques? I think you can make a decent argument that the practitioners of MMA have done so or are in the process of doing so.
Since I can't see much of a system in MMA.
Yeah ... that pretty much comes down to the limitations of your experience and perceptions.
it is just a simple and dumb brawl with people watching you, that's all.
Simple ... not really. There's a lot going on in high level MMA competition on technical, tactical, and strategic levels.
Dumb ... I suppose that depends on what you mean. If it's just a simple pejorative indicating that you don't like it, then fair enough, you're entitled to your opinion. If you mean that the fighters aren't highly skilled and knowledgeable martial artists, then you are sorely mistaken.