because I think WHMCS development often tends to be short-sighted, e.g we'll make this change now (increased frequency of cron jobs), but deal with the logical consequences of it down the road... happens time and time again.
there is the "Limit Activity Log" setting in General Settings, which I think by default is set to 10,000 entries...
but just checking my v7.5 dev tblactivitylog, there are 176,000+ records in there - of which, only 1,244 would meet the above condition.
when a cron runs, it's using an admin username, not "System" - so those entries would never be caught by this check...
would they even see it as a bug though? is the software doing something that it isn't designed/supposed to do?? (that tends to be their bug definition)... it's the opposite and needs to start doing something that it currently can't.. which whiffs more of a feature request to me - therefore, time to post an obligatory 5-year old feature request...
https://requests.whmcs.com/topic/ability-to-prune-system-activity-log