Young's modulus quantifies the linear response between stress and strain. If you look into stress strain data you always find that the linear relation (Hooke's law) is only valid for small strain. The more you strain your material the more nonlinear and irreversible deformation takes place which is simply not described by a linear relation between stress and strain. You will have plastic rather than elastic deformation and, after reaching the yield strength, ultimately your sample will rapture. Obviously this behavior is not included in a linear relationship. It therefore doesn't make sense to introduce a value for Young's modulus at such high strain.