Many managers and supervisors don't study management. This neglect does not stem any disdain for knowledge. It arises from a level of comfort. They often don't realize how much better they could be at their job.
Their organization judges them "good enough" (and even "outstanding") and they heartily agree, sometimes falling back on the "If it ain't broke, don't fix it" maxim. After all, they have plenty to do as it is. Why add management classes and books to their schedule?
They are getting by without realizing they are getting by. Their situation resembles that of a C student who is given an inflated grade and eventually mistakes C work for that of a B or an A. All will be well until the day that a higher level of performance is indeed required either in daily work or to gain a promotion and they run into a serious competitor whose scores were high and uninflated.
Most of us have, at one time or another, slipped into the "getting by" mode.
If we are fortunate, we know when we are doing so. If we do not know, some day we may be in for a big surprise. In today's ultra-competitive world, the individual is unwise to rely upon the employer's willingness to provide the training that is needed for the development of management skills. You are - you can groan now - your own brand and you don't want the product of You to be at the back of the pack.