Why dont they speak Austrian in Austria ?

They already have a language to speak. German.

Because it's a German speaking country.

The same reason Americans don't speak American, Australians etc.

Austria was once part of the German empire, then broke away to become a separate country. Google "Austrian history" to find out when this happened.

because most of the people that live there are germans

Because Austria was once the political core of the Habsburg Empire in the middle ages (until 1919 in fact) and the Empire contained within it multiple ethnic groups such as Czechs, Slovaks, Ruthenians etc. Now the dominant group (politically and territorially) was the Germans (these weren't Germans as we know them because Germany did not exist as a country until 1871 - modern day Germany was split into multiple states and principalities) who ensured that everyone within the Empire spoke their ethnic language; German. When the Empire collapsed, the ethnically german areas united and ensured their independence from the other ethnis groups and became a separate state; Austria.

And as for the name; Austria comes from the words 'Ost Reich' which means East Empire as the Habsburg Royal Family also owned Spain which was the West Empire.

I believe that austria was once a part of germany. but various countries speak languages of others. it usually goes back to the original settlers of that land

The same reason they speak spanish in south america??

We do have some Austrian words though! For example we say "Paradeiser" instead of "Tomate" (tomato) or "Erdapfel" instead of "Kartoffel" (potato). There are a few differences to the German German. It depends on the region where you live but in Germany most people don't know these Austrian expressions. There are quite a lot of Austrian words! ;)

Actually, Austrians do speak "Austrian", it is just not recognized as a separate language. I speak Austrian, and most Germans who listen to me will eventually say, "You're Austrian, aren't you?" When it comes to Spanish, we talk about "Cuban", "Puerto Rican", "Mexican" and so on when it comes to how people talk. Austrian is the same way. There is a famous saying in Linguistics: A language is a dialect with an army and a navy. What this means is that countries that are willing to fight for having a separate name for their own language (even if it is just a different dialect) usually get it. An example would be the Ukraine, which cared very much about having its dialect of Russian called "Ukrainian". These days, Austria is a very peaceful country that is perfectly willing to have others call its language German (but we Austrians know better!)

They do, lol
The written language is German, but also
here we have our own words and our own
dictionary.
And we speak different dialects in the different
parts of Austria, that can麓t hardly be understood
by people of other German speaking countries. ;)

there is no such thing as an austrian language, they speak german. likewise the swiss don't have a swiss language, they speak either french, german or italian depending on which part of the country they live in.

No more explanations - some good ones were already made.
Just a short explanation to the name 脰sterreich which comes from Ostarrichi and was first mentioned in 996 - long before the Habsburgs. It was then not a certain country or something but what people had named a certain landscape/area within the reign of Count Henry.