malospam, the reason for not being able to set mass is due to some history and practicality.
In UE3 it was possible to set mass directly. While this offers more control it has the problem that it requires the user to specify the mass for every object. We were seeing a lot of content where someone had accidentally left this value as default which would result in some really strange physics behavior (large dumpster or small box both had mass of 1kg.)
The intent of the new system is that you provide a mass scale which is used to determine the total mass. The idea here is that a car which is mostly hollow has a different mass scale than a solid rock. You can use physical materials to assign this mass ratio so that all rock like objects have their mass automatically computed without you having to enter anything in.
I agree that in some cases it would be nice to have complete control over the mass. Another thing to keep in mind is that concepts like moment of inertia, which depend on the mass distribution, are currently hidden from the user. If you specify the mass directly it would have to assume uniform distribution which might not be what you want (for example in the case of a can the mass distribution is on the edges of the object).
Hope that helps clarify things