# Fisher's LSD test

When we calculate Fisher's LSD test, why do we use Mean Square of Error which is the variance of all groups (pooled variance), as a variance of each individual group mean? In the denominator it's: sqrt(MSE/n1+ MSE/n2). Why don't we use group variance instead of MSE? Why don't we use a t-test just like the procedure of testing differences betweeen two sample means for independent samples?