In Which Scenario Share Object “mycustomobject__share” Is Not Available/created For Custom Object ?

Adding some code snippet:

There are a couple of reasons why there might not be a share object. First of all, if you have created a Master-Detail relationship on the entity, it disables sharing for that entity, since visibility is controlled by the parent. In this case, you would also notice that the Owner field is missing, since the owner can never be changed, and will always be considered the owner of the parent record. Secondly, it may be that your entity is in Public Read/Write mode. Sharing is only enabled for entities that are not globally accessible (i.e. they must be Read-only or Private).

Please check Setup > Security Controls > Sharing Settings for this entity's configuration.

Thanks