Different reasons for each type of car,what kind of car are we talking about?
The reasons range from abusive operation to poor design, poor materials, poor quality control upon assembley.
Ask the question about a SAAB and then ask the question about a SUBARU and then ask the question about a GM v-8 that recently had it’s heads off.
The gasket has to deal with high temps and high pressures, probably the most stressed gasket.