After the sentences you quote, the paragraph finishes with Mr. Collins might never make the offer, and till he did, it was useless to quarrel about him.

This explains Elizabeth's reasoning, and if Elizabeth wants to avoid a quarrel if possible, then not saying anything to her father makes sense. We have evidence from chapter 2 that if Mr Bennet doesn't do something Mrs Bennet wants (or in the case of calling on Mr Bingley, says he won't do something she wants), she gets angry and sulky and snappish, so if Mr Bennet was to say to Mr Collins that Elizabeth was a no-go zone, and Mrs Bennet found out, there would be a quarrel.

I think Elizabeth also intends to try to convey to Mr Collins that she's not interested in him, and thus hopes to fend off the proposal in a way much less embarrassing to all concerned than if Mr Bennet got involved. From chapter 17: ...she did not often speak unnecessarily to Mr. Collins... and she tries to avoid dancing with him at the Netherfield Ball.