Just to put a little balance on it - for some reason, some of us don't see this issue.
I've had many games with realistic balance with only the default playbooks. Some see too much passing (75+ percent) with the generic playbooks. After multiple tries, I constantly see 3 out of 4 plays passing, once 18 out of 20 plays, literally,, so I gave up on it. Just last night, played the Broncos post-Manning (and with the same coach), 40 passes, 19 runs with the Broncos book.
So, I'd see if you have the issue first. The OP does, so he should try it, but if you're getting balance, I'd leave things alone.
Just my perspective as someone who never had this issue.
And, no, there's no explanation (that I know of) as to why some guys have the issue and some don't. One theory is skill level (All-Madden might not have the problem, All-Pro might, customized or otherwise...though my CCM using a custom All-Pro set I'm trying to work on has no problems with default playbooks), there's reports that going farther in fran...CCM will cause the issue to fade away, especially as fictional coaches arrive in the league.
I keep asking to anyone who'll listen "Why do I NOT have this issue?" Have yet to get definitive answers, just the above theories as to possible triggers/causes.