Aside from the obvious risks of spending serious draft capital on high school arms, the college arm shotgun approach has the huge ancillary benefit that because these guys are physically developed, the bullpen is much more of a realistic fallback plan even with a year less before Rule 5 comes into play.
The high school guys are so much more likely to get hurt, or need physical development, or mechanical adjustments, or to find their control, etc etc etc. And the wrong injury can knock two seasons off the clock. There really isn’t a ton of time for an org to try to develop a HS guy and—if it’s not working—rebuild them as a relief prospect. Especially if it’s a guy like Ball or Groome that you signed for millions.
I was actually looking at some organizational k/9 stuff earlier today. Some examples of recent draftees (so doesn't include the Wikelman & Yordannys of the world)...
Christopher Troye, A+/AA - 17.7 k/9 (2021 12th rounder, $122,500 bonus)
Caleb Bolden, A - 13.97 k/9 (2022 7th rounder, $7,500 bonus)
Ryan Zeferjahn, A/AA - 14.59 k/9 (2019 3rd rounder, $500k bonus - Dombrowski)
Dalton Rogers, A/A+ - 13.58 k/9 not including today (2022 3rd rounder, $447,500 bonus)
Alex Hoppe, A+ - 13.35 k/9 (2022 6th rounder, $32,500 bonus)
Joey Stock, A+ - 12.79 k/9 (2020 undrafted, $20k bonus)
Maceo Campbell, A+ - 13.5 k/9 (2020 undrafted, $20k bonus)
Isaac Coffey, A+/AA - 12.13 k/9 (2022 10th round pick, $7,500 bonus)