Umm, they (mostly) do. Pyomo supports the AMPL .nl format, which means you can call any AMPL solver from Pyomo (most COIN-OR solvers have AMPL interfaces). The AMPL .nl format is the de facto standard for open-source optimization solvers because it's the only open standard, I believe [0].
So you would define your mathematical program in Pyomo (which isn't a terribly challenging syntax compared to AMPL). When you invoke solve, Pyomo will spit out an .nl file and call a solver (which reads the .nl file, solves the problem, and outputs a .sol file which can be read back into Pyomo).
Incidentally this is exactly how it works in AMPL as well -- all the interop is done through .nl and .sol files. I recommend reading [0] to understand how this works.