I am very ignorant so before someone tells me I don't know **** about football for saying this, just know I agree.
I don't know why it's so hard for rookies to catch on. Other than becoming a professional and all the other side things that being paid to play football brings, how much different is the principle? Learn the playbook, learn your job. Is that really so hard that rookies need a whole damn year to pick it up? Do they really need months upon months of offseason workouts that bad to know "when this play is called, I do this." I just don't really get it
It's difficult for a multitude of reasons, some players are use to being a star because put frankly they were genetically better than their opponents previously. In the NFL thats is rarely the case because in college you're one of 26,000 D1 players that season with a frame work of being between 18-24 generally. In the NFL you're one of 2000 including practice squad. So there is a lot less variance in physical ability compared to lower levels. Hence Harry's struggle to get open his size and lack of top end speed make it an issue although he can make contested catches it seems.
Previous years our O had the additional complexity of a lot of WR routes being options based on coverage/leverage, this meant the QB and WR needed to know the same set of options but also read it the same way too, the QB then needs to know your timing on a route to throw you open. Chad Johnson struggled because he could not read things the same as Brady, despite knowing the options.
There is also the need to understand a play holistically, not just what you have to do. An example would be two route: combo's double slant and slant out, in both cases Z runs a slant Y runs two different routes. The design of each play is to do very different things double slant create a vertical constraint on a #2 defender where slant out creates horizontal constraint. The Z needs to know that so they can vary their route based on defenders movement but also on how Y ran their route or if they contacted. A playbook is often 200 pages but shortened into a game plan but you need to know more than just your own route, plus the names its given and any hand signal for it that week. College playbooks are often simpler in concept and many players redshirt or see reduced time as a freshman which is not dissimilar to the NFL taking time to learn.
This is often why it's talked about as designing plays to get a player involved so they have less to learn and can contribute earlier. If our O was more west coast, air raid or air Coryell in it principles rather than Erhardt-Perkins it would be simple to integrate players but maybe less effective too. With Cam it's certainly simplified lets hope that means we can involve players quicker once they have the coaches trust in assignments. So sadly it's not as simple as "when this is called, I do this"
There are also in-depth details for TE, LB, C and S but my speciality knowledge is in QB/WR and I won't go into the mental aspect of everything in college planned for you vs pro you have to be in control. Unless you want me to write up about a typical week for a college player, I can't really do the same for a pro as a comparison.
This is pure speculation on my part: I believe BB holds our rookies back so they have earned their spot rather than it being a given for them which is something many won't have faced, I believe some fail at this test and struggle others come out much better players because of it.