Why weren"t they satisfied with their possessions in Anatolia
I think that far too many people view historical empires and figures using a modern day lens. They judge them based on the morality of the 21st century. Not realizing that such morality did not exist centuries ago. In fact, conquest was perfectly normal just over a century ago and it has been for most of history. To us, conquest might seem like an unnatural and evil thing to do. But throughout most of history it was expected and the normal way of things. After all, we are talking about the age of conquest.

The simple answer to this question would be that because they could. That is why the Ottomans conquered so many regions. Simply because they were able to do so.

Before the modern era, having control over more land meant having control over more resources. This meant a greater amount of wealth. Plus the larger the empire, the more the taxes. The more an empire expanded, the wealthier it usually became. Because of this and other reason, conquest was expected of an empire. There was nothing wrong with it.

The Ottoman Empire invaded other nations and conquered them for a similar reason. To expand and grow. The larger it became, the more powerful and wealthier it became.

The Ottoman Empire

As for why they were not satisfied with their possessions in Anatolia. One could take that question even a step further. Why did they aspire to be Sultans at all? Why were the Ottomans not satisfied as being simple Beys of the Kayı tribe? Why did the tribe aspire to be anything more than a Beylik in Anatolia?

Like all other leaders, the Ottomans wanted to conquer territory and expand their influence. They did so because, unlike the others, they had the capability of doing so. That is the only thing that stopped others from doing the same.

We could ask the same question of Rome. Why were the Roman people not happy as a simple town? Why did they want to expand and form an empire? Why did they desire power and wealth?

In the age of the Ottoman Empire, conquest was normal. It was actually expected of every empire. When an empire stopped expanding, it was considered to have started to stagnate. Like all other empires, the Ottomans expanded their holdings till they ruled a massive empire. This required them to invade other nations and kingdoms to subjugate them as vassals. The only difference between these smaller kingdoms and the Ottomans Empire was the capability and power of the Ottomans that allowed them to conquer.

One could ask why the Romans were not happy as a little town. What made them conquer and expand to form one of the greatest empires in history.