Why do reactions reach equilibrium?

If the forward reaction causes an increase in entropy then the backwards reaction causes a decrease in entropy, so the backwards reaction is non-spontaneous. If so, then why does the backwards reaction begin to happen spontaneously and an equilibrium position is reached? I get that when you have mixing you also have a higher entropy because there are more microstates available in the given energy configuration, but why is an equilibrium position reached if the backwards reaction is supposed to be non-spontaneous? And why not just get a 50:50 mixture of that's what will increase entropy.


P.S.
1) I do A2 chemistry.
2) Graphs are more than welcome.
3) Arguments in terms of disorder are not preffered, but please use microstates and 'ways to arrange energy'.


