Maybe for the same reason that you can by a CPU without a heatsink - in order to provide flexibility. They assume that the number of people that will want to sp

Maybe for the same reason that you can by a CPU without a heatsink - in order to provide flexibility. They assume that the number of people that will want to specialize and customize their heatsinks/fans will be so high in proportion to the people that will be just fine using the standard sink, that it is not worth it. There are very cheap easy to mount (like 2 seconds easy) heatsinks that will do the job (probably with thermal throttling though).