While somewhat true, I would caution about how much a HS coach actually sees. If you just do the math, coaches have very little time before games begin to evaluate players. Therefore, you could be looking only at 20-30 hrs. of practice time. With maybe 20 kids on a team, that is at best about an hour of actual skills time. And in many cases, the penciled in starters will get more time than the backups.
Not sure what kind of practices you're used to, but they sound crappy. I would venture to guess that in 30 hours of practice (not factoring in scrimmages and games) my varsity kids have all gotten near 1000+ groundballs/flyballs and 1000 + swings. A lot of those on the field "game-like" reps. Subvarsity kids not identical numbers, but not too far behind.
While I am sure they are getting good, solid, reps, the amounts just don't add up. If you take your 9 starters and give them 1000 fielding reps and 1000 hitting reps, that's 18,000 total reps over 30 hours. That's something like 600 reps per hour. Unless you have a lot of coaches and resources and can break up into many stations, you're probably doing something significantly less than 1000 reps per player. Especially for the backups (probably another 5-10 kids), who probably get less than the starters.
At my son's school resources are tight. They only have two tunnels. Varsity monopolizes them. So the lower levels rarely get to hit on a regular basis. With a single coach, situational infield/outfield is a slow process.
Not to get bogged down into calculations and such, but the reality is the backups are getting less and less opportunities to prove themselves. They just need to be ready when called.
We do 4-fungo infield 3 times a week for 30 minutes at a time. There are variations to it, but each position gets roughly 400 groundballs hit to them a day (we don't slow down swinging or we use machines/fungoman, so about a gb every 4 sec or 15 a minute for calculation). About 5 deep at every infield position (sr - freshman), that's about 80 gbs a day or 240 a week at the bare minimum. 30 hours is over three weeks of practice for us (only 8 hrs per week per UIL), so you can see we're close to or past the 1000+ mark I referenced off the cuff, and that's just in our one drill. ANd most of those are quality reps with our kids.
If you mean 1 coach with one fungo and 9-kids on the field than you have a point, but like I said, that's a very crappy practice.