You're making this sound far more simple than it really is.
In 2018, our two biggest needs going into that draft were QB & LB. The Bills made the moves and got the players they wanted to fill those positions. Now one pick has worked out much better than the other, but at the time both picks made sense.
Also, those two other defensive players turned out to be Harrison Phillips and Taron Johnson.
Then in later years the Bills drafted Cody Ford, Devin Singletary, Dawson Knox, Gabriel Davis, Isaiah Hodgins, & Spencer Brown.
I understand that not all of these players have worked out, but a good amount of them have, and it shows that the Bills do address the offense in the draft.
Taking a more heavy offensive approach in how we draft, might be the way to go, but it's not as if the Bills are completely neglecting Josh Allen.