Question: As far as I understand it, an illegal contract is void, and the Polish Supreme Court has assessed that EU contract violates the Polish Constitution.

Why is Poland still considered an EU member state?

Answer: 
As far as the **EU** is concerned, Poland will remain a member until it triggers [Article 50](https://en.wikipedia.org/wiki/Withdrawal_from_the_European_Union#Procedure) and the time limits run out (or a different date is mutually agreed). Article 50 leaves it up to each state to set the constitutional requirements for the Article 50 notification.

Hypothetically, the Polish supreme court could judge itself qualified under the Polish constitution to write and deliver the Article 50 notification, but it has not done so. If that were to happen, the EU would have to decide if they take such a letter by a court rather than a government at face value. The supreme court could also order the government to write the Article 50 notification. If the Polish government were to comply, it would be closer to the expected process.

But politically speaking, neither is a realistic option. The Polish government and their court do not want to leave the EU, they want a maximum of EU subsidies with a minimum of EU interference in their traditional values, and they want to win certain voter groups with their rhetoric.
This is a large question but part of the answer is that not only is EU Law in conflict with the Polish Constitution but the two systems of law disagree as to which system of law has priority.

According to EU Law, EU law is supreme and the courts of Poland are duty bound to disregard the Constitution of Poland where it conflicts with EU law. So on this basis the EU Treaties (what you call the EU contract) is not illegal and Poland is still a member state.

According to the Polish constitution as interpreted by the Polish courts, parts of the EU treaties are contrary to the Polish constitution and therefore the courts of Poland are duty bound to disregard the EU treaties insofar as they conflict with the Polish constitution. As I understand it the Polish courts have not declared the EU treaties completely illegal - just certain parts of them. On this basis Poland is still a member of the EU but just not bound by some of its rules.