Well they can't just 'go suspect'
Suspect mode requires that SQL encounters corruption during a transaction rollback or during crash recovery. Typically, because the system databases (other than MSDB) aren't changed much, there aren't many rollbacks and corruption would much more likely be encountered during crash recovery on startup.
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
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