If you read anything about how the Windows installer system works, it's obvious they applied some ideas from transactional databases to program installation and maintenance, not to mention the
.msi files themselves are a database.
There is always the question in designing any database - do you want speed or accuracy/safety? Given that installers can modify system configuration and that a mishap could render the system inoperable, safety has been given a priority over speed. One of the reasons why
.msi installers are so slow is because rollback files are made for each file, etc. that will be modified, and then deleted afterwards - allowing any changes to be "rolled back" if something goes wrong in the middle of things (such as a power outage or system crash).
Now, I believe the MSI engine itself enforces installing, modifying, or removing only one program at a time - if you try to run an
.msi while another is uninstalling, for example, it either won't run or will wait for the currently running uninstall to finish. Non-MSI installers may not behave this way - since they don't use the MSI engine. But because of this safety design decision, this is probably why
appwiz.cpl insists on only letting one uninstaller be called at once.
CCleaner allows you to kick off uninstallers without waiting for previously running ones to finish. MSI installers will likely still not work in parallel due to the above.