Because you want to be maximizing your EV on each street, not trying to make the hand 'easier' to play. If you raise for information and someone calls instead of folding, his range is stronger. While that might sound like a good situation to be in because you know your marginal hand is now nearly always behind, a more +EV situation would be to call and keep in the weaker part of his range so that you can still get value on later streets - you want to be playing against ranges which you mostly beat but sometimes lose to, rather than narrowing that range so that only hands that beat you call and weaker hands fold and don't lose any more money.