Some Prophecies are Conditional.
Prophecy is giving a message from God or the Lord Jesus to another person or persons, and that the main reason for prophecy is God’s love for us.
We now need to examine an aspect of prophecy that is very important, and not well understood by most Christians: when a prophecy is spoken to people about them personally, it is almost always conditional in nature. The fulfillment of the prophecy often depends on the attitude and actions of the one to whom the prophecy is given. This is different from the generally accepted teaching that when God declares something, it is “set in stone,” and will absolutely come to pass. If you are one who has believed that a true prophet of God is defined by the fact that his prophecies always come to pass, you should carefully read this.
Certainly there are prophecies from God that are “set in stone” and will come to pass exactly as God spoke them, but not all prophecies are that way. Prophecies spoken to individuals or groups about what will happen to them in the future are usually conditional upon the actions and attitudes of the people addressed. God gave us free will, and He responds to the free will decisions we make. What makes the prophecy conditional is there is an unspoken “if ” in the prophecy. In other words, the prophecy has a condition in it.
Take the prophecy Jonah gave to the people of Nineveh: “…Forty more days and Nineveh will be overturned” (Jon. 3:4b). Anyone reading the book of Jonah sees that Jonah’s prophecy did not come to pass. Nevertheless, Jonah was a true prophet. Furthermore, had the prophecy been fulfilled as spoken without being conditional, then what would have been the point? To tell thousands of people they were going to die in 40 days? No, there is another explanation.
The prophecy of Jonah, and other prophecies to people, often have an unspoken “if” clause. When the “if” is expressed, the prophecy looks like this: “Forty more days and Nineveh will be overturned if you do not change.” God is expressing His love via a prophecy that calls people to greater things. The prophecy gives people a chance to rise up and walk in a godly fashion before Him. The people of Nineveh did, and therefore God’s prophecy was not fulfilled. Of course, sometimes a prophecy of good is spoken, but the people turn to evil. Those prophecies are also conditional, and the good that was foretold often does not come to pass.
Jeremiah Sets Forth the Conditional Nature of Prophecy in Jer.18: 1-11

1. The word which came to Jeremiah from the Lord:
2. Arise and go down to the potter’s house, and there I will cause you to hear My words.
3. Then I went down to the potter’s house, and behold, he was working at the wheel.
4. And the vessel that he was making from clay was spoiled in the hand of the potter; so he made it over, reworking it into another vessel as it seemed good to the potter to make it.
5. Then the word of the Lord came to me:
6. O house of Israel, can I not do with you as this potter does? says the Lord. Behold, as the clay is in the potter’s hand, so are you in My hand, O house of Israel.
7. At one time I will suddenly speak concerning a nation or kingdom, that I will pluck up and break down and destroy it;
8. And if [the people of] that nation concerning which I have spoken turn from their evil, I will relent and reverse My decision concerning the evil that I thought to do to them.

Many people think that the lesson the potter and the clay teaches is that God can do anything He wants to with people. However, that is not the case. In fact, the lesson is actually the opposite. The potter knew what he wanted to make from the clay, but when he tried to make it, it was “marred” in his hand (v. 4). The clay was uncooperative, and would not take the shape the potter had in mind. The potter did not mar the clay, it was marred in itself.
People and clay are a lot alike, and God, the ultimate potter, works with us like a potter does with clay. God has an idea for our lives, but often we are uncooperative, and will not take the shape He desires for us. What does God do? He does His best to work with our free will and make something different out of our lives. The reason God had Jeremiah go to the potter’s house was to teach Jeremiah that the way God works with people depends on what the people are willing to do.
HAPPY SUNDAY.

I keep wondering why we have false prophets

Who gives prophets their prophecies? Is it not the Lord? Is God man that he should lie? No. Prophecies are not conditional. If they were, what then did the bible mean when it said "when a prophet speaks and his words do not come to pass, he spoke out of his own will and not of God and we should not acknowledge him as a prophet"?

His words are yea and amen. That is why when a self acclaimed prophet speaks and it does not come to pass, his followers should understand that he spoke not of God and should find an actual prophet. Rather than encouraging him in his ignorance and giving excuses for him.

My reply to your reference to Jonah's case is actually stated in the bible. Jeremiah 18.... The lord tells us that if a nation should repent of their sins and follow him, then, he shall not destroy them as he planned. That was why Jonah's prophecy did not come to pass, because the people heard and repented.

Search the scriptures.