The reason I believe is because they are artificial elements and so an atomic mass cannot be given. This is because the atomic mass is given by the

Quote

weighted average abundance of natural isotopes that occur in the Earth's crust and atmosphere. For synthetic elements, the isotope depends on the means of synthesis, so the concept of natural isotope abundance has no meaning. Therefore, for synthetic elements the total nucleus count (protons plus neutrons) of the most stable isotope, i.e. the isotope with the longest half-life—is listed in brackets as the atomic mass.

That being said, this does not mean we do not have a clue what teh actual masses are since for the case of the 272Rg and 279Rg, you can see that the masses are known [en.wikipedia.org].

To find the measured or otherwise determined "real" mass of isotopes, it is often a case of looking up the nuclear physics literature, or always a good port of call is the CRC Handbook of Chemistry and Physics which is updated regularly (it is currently in its 92nd edition).

there is also the confusion of different forms of atomic masses, and I submit this for inspection [en.wikipedia.org]

As for calculating masses, no it is not that easy, especially for large nucleon numbers. The binding energies (and hence overall nuclear mass) is a quantum mechanical calculation, and depends upon the number of protons and neutrons in the nucleus, and how they fill their energy shells, as well as the balance of electrostatic and strong forces. It is not simple as my basic equation suggests, which was just used to illustrate how cobalt has a larger mass than Nickel, even though Nickel is higher in the periodic table than Cobalt. There is a semi-empirical method of calculation that uses what is known as the "liquid drop model" which you can read about here [en.wikipedia.org].

Jonny