i dont understand why it is so difficult to get a block when there are thousands of blocks available
There is one block that will be solved in the next ten minutes (on average) and it is worth about $400 at current exchange rates.
Tens of thousands of miners globally are performing 12,960,000,000,000 hashes in the next second and another 12,960,000,000,000 hashes being performed the second after that, and so on until a hash is found that matches the criteria specified by the difficulty level. When that happens, the current block is solved and the next block begins. Difficulty will increase automatically every so often so that blocks are solved at a target rate of one every ten minutes.
That's the technical reason.
If you instead are asking what is the purpose of this hashing, it is the method bitcoin implements to make it so that the blockchain can be trusted as being correct. Since there is no central authority, this "proof of work" system is what ensures that trust is not misplaced.
