I would like to add few more points to your discussion. Mainly because of the type of the job, some of the executives do not act as project sponsors.
In large companies, top level managers are very busy with their works and they do not have proper time to sponsor. Due to this sponsorship falls down. I believe that getting an executive and involving them in the project is very tough job. Projects will have very high rate of success in the presence of a sponsor with their support and commitment towards the project. However, a sponsor is the key person who plays an important role in the success of a project.
If an executive is forced to work as a sponsor, then achieving this level of commitment towards project may not be possible.Success or failure of the project in the organization depends on the team members, manager, environment and the processes followed in the work of the project. Sponsor may not be the responsible person for the success or failure of the project because even though he plays a key role in the project but he will not be working with the team directly. Thus he was unable to monitor the process of work in the project.
However Sponsor is the person who make decisions regarding the project.