Someone who is rated an 82 is listed as a three star recruit. So is someone who is listed as an 88 or 89. There's a world of difference between those types. And you'll also note how rare it is for someone rated lower than 88 or so to be drafted.

And you're right that not every highly rated QB becomes a stud.  But the odds are a lot better.  Here's the last decade of recruited BYU QBs, broken into two groups

Top 1000 overall recruits in their class

• Jacob Conover (2019, #317)
• Jaren Hall (2016, #712)
• Zach Wilson (2018, #958)

Not in top 1000 in their class

• Sol-Jay Maiava (2020, #1028)
• Enoch Watson (2024, #1222)
• Carson Su'esu'e (2024, #1236)
• Jake Retzlaff (2023, ~#1250)
• Noah Lugo (2024, #1391)
• Beau Hoge (2015, #1498)
• Ryder Burton (2023, #1538)
• Stacey Conner (2018, #1830)
• Baylor Romney (2015, #2030)
• Hayden Livingston (2016, #3036)
• Joe Critchlow (2015, #3174)
• Luc Andrade (2019, #3521)

Kedon Slovis was also #689 in his recruiting class and then rated higher as a transfer so probably falls into the first group. 

Being in the first group isn't some guarantee of success (see Conover). And being in the latter isn't a death knell (Jake Retzlaff and Baylor Romney have at least been quality backups)....but the odds of a quality starter are a LOT higher in the first group. Ratings do matter