It doesn't matter how brilliant your subject knowledge is, you could have a double first from Oxford in English or Maths. As an teacher you have to know how to break it down to teach children who
a. have emotional, social and behavioural difficulties and get easily distracted
b. children who do not speak English as a first language
c. children with learning difficulties
To name but a few.
The subject specialist area is the teacher's responsiblity. We have to learn the subject to the appropriate level that we teach. We are constantly improving our knowledge hence continual professional development.
When children are not engaged in the learning, they will begin to do off tasks activities such as talking amongst themselves, throwing things around etc. There will be no learning taking place unless you learn how to create engaging lessons and have brilliant classroom management skills. Children these days can't listen for more than 3-5 minutes, and most learning, is task and independent learning based. You have to plan effective tasks to lead to a learning objective. You are assessed on how well your children are learning. There is nothing easy about it. It is totally different to how I was taught at school and takes years to master.
If you do not know the subject well enough, you would do well to either avoid teaching it or use up all your spare time to learn it. I have a Masters in my subject specialism and there are still aspects I do not know because all subjects are broad and all degree curriculums vary. I am not a history teacher but you could have studied Modern European History for your BA. You may be expected to teach perhaps The American Slave Trade, The Tudors, The Vikings etc. You have to find the resource and learn it to the level required. If you have a degree in the subject, this is easy but still time consuming.
Primary teachers have to teach subjects to around a Level 4 which is equivalent to GSCE Grade D. They always have qualifications beyond this, i.e. GSCEs, A levels and a degree. They have to learn their subjects to around GSCE level and then break it down to whatever age group they teach. Again, they can do this, but it is time consuming.
These are some of the skills the primary and secondary training gives i.e. how to break it down to the appropriate age group and a structure for developing subject knowledge. Many teachers spend their holidays learning a new topics. It is ongoing.
Being a teacher is being a perpetual learner. You have to really enjoy learning to teach.