Well they are still babies who need alot of attention. I really like keeping my girls for as long as possible with the mom as they still learn so much from her. But yeah since the boys have to be separated that part doesn't help them much anymore.
But it is still a very crucial point in their little rat lives where they are learning how to be a rat and how to be social. Even just being with their sibs teaches them alot of rat behavior. I like to compare it with puppies who learn important social graces like bite inhibition. I think it also helps them be more social towards other rats which will lead to easier introductions in the future.
As caregivers I also feel it is our responsibility to make them very well adjusted- again such an important time in their life. It is great to introduce them to alot of different circumstances. Different people, noises, different types of rooms, scents, toys, wheels, how to climb, free range, etc... making an all around well adjusted bomb proof rat!
Plus getting taken away from your home early when u r still alittle baby can be scary. While they are away from their mom, they are still used to this as their home and their brothers and you.
There is also some safety reasons. While it is very possible to find a really awesome home with experienced rat owners, some ppl may never of had rats before or not be experienced with babies.
Those few weeks babies grow quite alot!
What are they going to feed them? Having a high quality diet with extra protein is very important to babies growth.
If someone has other rats, usually at 5 weeks babies are still small and not yet mature enough to really be housed with adult rats. They could easily be injured, bug the adults and not understand when the adults try to put them in their place leading to injuries as well.
Rat proofing and cage bars need extra thought for smaller babies. Alot of ppl do not think of this or think it will be ok.
It is up to you ofcourse. But that is some of the reasons I like to adopt later rather than sooner.