[smilesnd]

The simple answer is because Elasticsearch has had thousand of hours already put into writing its code base. The real question should be why shouldn't you use Elasticsearch? Is the code base to large to fit where you need it to be? Will it be able to scale with your project? Is it efficient enough for your requirements? When looking to use a piece of technology the requirements and long term effects are what matter. Roll your own if that is what is required for you to reach your end goal.