Why is it bad to block check with pawn?

I don't know exactly, but for many cases, I see players block check by moving their knight to block it. But:

Blocking with a piece, develops a piece.

And most people do not like to block c3/c6/f3/f6 with a pawn, because they usually want to put their knight there.

In the first example, blocking with the pawn is clearly superior to 3...Nc6

The second example is a bit weird but again, blocking with the pawn would be good - though white would have a large advantage after any sensible blocking move.

Blocking with a pawn to gain tempo is often desirable, but when it blocks a desirable development square for another piece (here the knight), there could be a tradeoff. Plus, when the pawn structure is looser, blocking with a pawn could be fatal:

This is a contrived example, but in general, the facts that pawns cannot move backwards and are immobile, potentially getting in the way of other pieces, are the main reasons to not block with a pawn. You will want to block with a pawn if doing so gains a tempo without compromising your position.

In both of the examples given in the first post, blocking with the Pawn is fine.