5000 feet is a safe altitude to climb to that will keep you clear of arrivals in their early descent area. They can be descending to 6000ft either just east or west of the field depending on the configuration. Aircraft have been known to turn on course sometimes without being told on takeoff and if you are climing to 7000 ft then you are smack in the way of arrivals.
Also given the strong headwinds that YYZ gets at times some of the biz jets are level at seven shortly aftertakeoff, once more in arrival airspace.
It is useful for the departure controller to have some altitudes available from 5 throught to 7. If all all SIDs for the jets went to 7 we lose that tool. In situations with prop and jet turns we at least have 5,6 and 7000 feet available to us if we need it.
Low performance arrivals also come into YYZ at 6000ft ie the C208s. In worst case scenario...if there was a radio failure on departure....5000ft keeps you underneath these arrivals.
It may seem like 99% of the time that you are being given the climb instantly to 7000, but the 1% of the time you are not is the reason why they won't be changing the SID altitudes any time soon.