Steel reinforcement material commonly called rebar is typically more expensive when compared to the cost of the concrete material that the reinforcement would replace. So, the cost of the entire slab would be higher.
Adding too much reinforcement such as steel rebar will actually make the slab more flexible (Less rigid) which may not be what you are trying to achieve. Any increase in the compressive strength of the slab may not be what you want or need.
The added rebar may be too close to the surface and thus not covered by sufficient concrete material to prevent the concrete from cracking at the location of the rebar. This would require that a thicker slab be poured to cover the extra rebar.
The specification and design of a concrete slabs have been optimized for their intended use. Go back to describe the design specifications in terms of how strong and flexible you need the slab to be. And then do not over-design it.
Fallingwater, the famous house designed by Frank Lloyd Wright suffered from having too my rebar installed in the concrete slabs causing them to sag.