Y do my knees keep buckling and give out?

Giving out. There is true instability (giving out/buckling) related to a ligament injury. This can be the anterior cruciate ligament or the medial collateral ligament. A good physical exam by a trained physician usually diagnosis this problem and it can be confirmed by an mri. Instability can also come from weakness related to pain from arthritis and the quads muscle group being inhibited.
Knee buckling. There are various reasons why this may be happening to you. If you had an injury there may be a ligament tear in your knee. Other reasons may be muscle weakness or malalignment of your joint. Get evaluated by an orthopedic surgeon.