How is presupposition of the order of reaction justified in Michaelis-Menten model?

from what I have understood, to derive Michaelis-Menten equation, we first suppose that


$\ce{E + S <=>[k\_1][k\_{-1}] ES->[k\_2]E + P }$


and make the 'steady-state assumption', that $k\_1[E][S]=(k\_{-1}+k\_2)[ES]$.


But from what I've learned, the order of reaction cannot be derived from its equation, but can only be decided by experiment. How can we 'presuppose' the order of these, especially when both $E$ and $S$ are macromolecules? The left side might as well be $k\_1[E][S]^2$, for example.


What am I missing out?


