You can see in the photo that the valve spring seats are too large for stock valve springs. A stock single valve spring with a damper is 1.265" O.D. to 1.300" O.D. and those valve spring seats are at least 0.200" larger in diameter.
A spring that dances around on the seat at high RPM due to spring seats that are too large will fail. That is probably the #2 reason why valve spring fail. The #1 cause is coil bind or inadequate assembled height for the valve lift your cam has.
You may have both problems if you do not know how much valve lift your camshaft has.