Because there is none. So you hedge with wanting to have more talent at QB, and experienced talent, rather than less.
And even if Taysom only makes it 4-5 games, you then have Tanner there to take over.
With no Taysom, if Tanner goes down at game 4-5, you then have Hoge or Detmer, and the season become brutal really fast.
Since 2009, BYU has only had 1 season where they did not have a QB go down, and that was a Taysom season.