Whilst this is a reasonable request, this isn't going to be changed now for the 5.0 release now, but may be something added for the following release. I would recommend that you raise a JIRA feature request for consideration by the Product Management team. Alternatively you could implement an extension to meet this requirement and look to contribute it back. The Sites Manager has been implemented by the new Aikau UI framework and it should be relatively straightforward to add in search capabilities via the existing Search API (however, be aware that there may be limitations with the existing site search API as to whether or not a Site Manager can find private sites that they are not a member of - this would need to be investigated)
Regards,
Dave