The problem for most C and D players, myself included, is that they (we) have the belief that every new rack, every turn at the table is an opportunity to run out, in spite of a history of not being able to go beyond 5 ball runs in 100 tries.
Like Brian at FX Billiards says in one of his training videos, as a beginner or intermediate beginner, running out 3 balls is hard. Practice 3 ball runouts. When you can do 10 in a row, go to 4. It’s harder still. He says, and I agree, once you get to consistently running 6, improving to 7, 8, and 9 is a shorter trip.
Most of my runs fail beyond 4 balls because the further into a rack, the more off line the cue ball gets, resulting in failed hero shots to maintain the run.
The key is to have realistic expectations and play safe probably well before the ego says you should.
I think it's the opposite, going through the first part of learning to mid level play is pretty quick, given someone has learned good mechanics. It's the tip of the pyramid that takes the most time to reach. There are many players that hit say an APA 5 or 6 and then stop there, because they don't have the consistency and experience in getting through the rack properly. To max out APA or another league and to go over that, that is the part that takes years of experience. I know several players that started a year or less ago and are close to running out racks, they have most of the skills in shot making and moving the ball, but they don't have the knowledge yet of what the right shot is or the steadiness of experience in situations or how table conditions can affect your shots. It's the same in anything, progressing through the lower levels is easy, getting to the top takes 90% of the work and time. Look at something like a car, it's easy to get a car traveling say 100 mph, but the progression from 100 to 200 mph is much harder than the first 100, and the higher you get, the more effort it takes to get there, the limitations curve is steeper the closer to the limit you get for anything. That's why there are so many average athlestes and equipment and so few top ones that use the best equipment. You can buy say a $1,000 bike that is very good, but the levels that you would see on an Olympic track or Tour de France are many times more to get even an extra 20/10/5 % better, you spend 10-20-30 times the cost and effort to get to the top of the pile.
Almost everyone I see that is not able to run out a rack even in many tries, is stuck there due to mechanics or some mental nervousness thing that just blocks them from executing. Those that can run out a rack every so often or most of a rack, that can't beat the better players in the room, are at that level due to lack of experience in situations they end up in and picking the wrong shot to play. I have been stuck playing someone just ready to go rack for them, and they pick some shot that is less than ideal and end up giving me the win because of their mistake in planning the rack. My thinking goes from "this is an easy layout they should win" to "hmm, wonder why they played that ball instead of this one, I guess I have a chance since they just lowered their run out chances by half".
Of course, this is general statements, but ones that I have found to be true after watching and teaching all sorts of players. Going by APA skill levels, if someone is a 1/2/3/4, they are there as either new players or because of bad mechanics, if they are a 5/6, that is mostly due to less experience not so much shooting properly, although that may also be due to flawed mechanics where their shots break down but just less often. Another reason a player may be stuck at a lower level is due to not playing outside of a small circle, say if one only plays in APA matches a few times a week and never sees any good players, or is not interested in learning or broadening the pool world they know about. A lot of league players have no idea what the "real" rules of pool are or just how good the good players are, their gods are APA 5s, 6s and 7s, who are in turn easy wins for any decent tournament player.