Because you don't build a program by acting impulsively and focusing on one season at a time.
There's a concept called delay discounting, which is essentially a way of looking at how big of a difference in reward do you need to push back when you would receive it. The easiest way to look at it is is time x money. So would you prefer a dollar now or 10 dollars next week. You can manipulate both the time and money components of it.
https://en.wikipedia.org/wiki/Time_preference or
https://en.wikipedia.org/wiki/Delayed_gratification
So for lack of a better index, think of this like madden ratings for the players. Lets imagine player 1 is a 78 (a senior with no redshirt to burn) and player 2 is a 79 (a Freshmen with a redshirt to burn). Would you rather have the change in 1 improvement now or another season when the freshmen is at their best (maybe an 89 Senior) but that's 4 seasons away?
The team is essentially doing this math based on the differences between players now, how they project the players, the current other depth on the roster, etc.
It's easy for us as fans to say start the best player no matter what. Personally, I'd rather have the best product a few years from now.