Why are post hoc test preferred over several t-test

Almost all the post-hoc paired comparisons are variations on t test, but basically the difference between post hoc paired comparisons from t test is the way that the standar error is estimated. On t test the standard error between two groups is easily calculated, but what happen when you have three or more groups? Then you must to perform an ANOVA test, and this gives you the answer to the standard error: the mean square of error (within groups) can be used to estimate this, and all post-hoc procedures uses this value in its calculus.