Simple concrete is enough in almost all cases for what we do - if we use enough of it. Rebar allows dramatically reducing the amount we use in most cases, with this predictable trade off
>>lazide+(OP)
Plain concrete is simply not allowed by code for many applications. So even if you could theoretically justify the design, you couldn’t get the permit to build it.
Physics, economics, and bureaucratics is a good summary of structural engineering. The last one can’t be ignored.