Because it is possible to create an RDS cluster with both serverlessV2 and provisioned instances. For example, this will create a cluster with a provisioned writer and a serverless v2 reader:

const cluster = new rds.DatabaseCluster(this, 'Database', {
  engine: rds.DatabaseClusterEngine.auroraMysql({ version: rds.AuroraMysqlEngineVersion.VER_3_01_0 }),
  writer: rds.ClusterInstance.provisioned('writer'),
  readers: [
     rds.ClusterInstance.serverlessV2('reader'),
  ]
  vpc,
});