Because at full employment the aggregate output of society, and by extension also (Marshallian) welfare is sustainably maximized.
If we produce below full employment that means we are not fully exploiting some scarce resources. If we produce above full employment we are exploiting resources in unsustainable manner (workers working overtime to point of exhaustion, running machines more than optimal etc).
Also in simple Keynesian models optimal rate of inflation is actually zero. Mind you this only holds for very simple models. In a non-linear New Keynesian models with zero-lower-bound there will be usually some non-zero optimal inflation. Although, the optimal rate of inflation will still be close to zero, it could be as low as 0.3% (e.g. see Gorodnichenko et al 2011).