Perhaps because you used the word "mass" which is invariant, and not "relativistic mass" which is not a measurement of mass at all, but of total energy of which mass is a part.
You mean rest mass right?
There is relativistic mass and rest mass.
"mass" can refer to either, with the meaning varying depending on context.
There is only one kind of mass and it is invariant. "Relativistic mass" is a measurement of energy, not mass.
Energy has inertia and exhibits gravitation and interacts with gravitational fields. It contributes to the SEM-tensor.
A photon never has mass, but it has inertia, momentum, and exhibits gravitation. It contributes to the SEM-tensor.
A closed pot with water has a given invariant mass of metal, water, and air. If we heat the water to a boil it increases the energy of the boiler, but not the mass. The boiling pot of water (were we able to accurately and precisely measure it) will have greater inertia, momentum, and exhibit stronger gravitation than the same pot at room temperature. The same pot on a moving train will have greater still, because it has added kinetic energy.
At no point, in any frame, does the mass of the pot change.
Since we classically think of inertia, momentum, and gravitation as properties of mass and not energy, this measure of total energy (including mass [mass-energy equivalence]) was termed "relativistic mass". It becomes frame dependent (relative velocities adding energy, and thus inertia, momentum, gravitation), so it is "relative". But it is not a measure of mass.