Two reasons. First, Donald X. is now avoiding "while this is in play" wherever possible. And unfortunately, that's everywhere! These days, if a card won't work without "while this is in play", that card probably won't get to exist. Luckily I don't think that's yet caused a card to be cut during testing, but I could be misremembering. Anyway that's why all recent effects like this have been "this turn".
Second, in the case of Inventor (and Priest), I think the published wording is probably clearer. I think your suggested wording would send most players to the rulebook, wondering whether the cost reduction applied to the gain on the card. I get that it's elegant, but elegance does not always equate to clarity.