Because they take up a lot of space in Navigator.
This appears to be an intermittent problem when moving designs up to newer versions of the software. One method to remove them is to change the current ripper to a custom ripper and run Update other objects - bus rippers, or the other way round if you already use custom rippers, change them back to default rippers. This is done in Settings - Special Components by either adding a custom ripper or removing an existing ripper.
Before you do this though you should consider the impact of doing so on your design, even if you change them back after the switch. If your design is large and complex you may need to carefully check the results of any change.
So far we have not tracked down what causes this in the tools as we do not have a reliable way to reproduce this.
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