Spindler and Fisher were multiple year starters in high school, and both have real experience when it comes to the complex zone blocking schemes that we use. From what I've seen, most high school linemen are only familiar with man to man blocking.
There's also a big difference between the learning curve for a true freshman quarterback versus a lineman.
If the COVID-19 situation hadn't robbed Buchner of his senior season, then he could have had a more realistic chance of starting, but since we have a polished veteran ready to play right now, it makes more sense to let Coan start the year.