As someone who has spent the last four years teaching at a private elementary school...
The major difficulty facing elementary teachers, and specifically those who teach the lower grade levels, is that you are often working with developing a person rather than a student. Many first graders come into the school with asymmetric development and skill sets. A child that is an excellent reader may have terrible fine motor control and thus takes literally 10 minutes (or more!) to write a simple sentence from a board. Or conversely a child who can barely read, even in their second or third year, may simply have no interest in academic pursuits and be chomping at the bit to get outside and run or have the opportunity for gym class. There are often behavioral issues at hand - a child may be much too upset to even bother paying attention in class because the teacher told them that toys/stuffies must stay in their backpack or be taken away (for example). Or unfortunately many young children are affected by poverty or divorce or unsafe housing and thus are affected by their home life to such an extent that school isn't even on their radar. Developmental and mental issues are also very potent in this stage of life. The social determinants of education are extremely important in the younger years.
I think everyone here also seems to vastly overestimate the pacing of elementary level education. Seven years is NOT a long time to introduce scientific concepts to children. You are literally introducing the world to many children all at the same time and this naturally requires you to move at an appropriate pace. You are also limited by curriculum battles, the economics of classrooms resources and material management, parent interference, the interests of the children, and most importantly - the all important standardized testing. I am very fortunate that our school has literally zero standardized testing and we are free to develop and pursue any area of the curriculum as we see fit. But between music, art, English, French, PE, mathematics, zoology, botany, history, sociology, creative writing, volunteer activities, and all the other things the children have to do, some areas will naturally fall behind - especially if the child is simply not interested. For example, how many grown-ups who despised algebra actually remember it?
All grade levels suffer from the generic problem of low-quality teachers, and I've found that generally excellent teachers are more commonly found in the younger years as these people get involved in teaching specifically because they enjoy teaching rather than simply enjoying the subject they teach (high school teachers tend to suffer this affliction). A particular sore point of mine is that in my country (Canada) it is FAR too easy to become a teacher and we have a serious oversupply of people with B.Ed degrees who have teachables in English and history or biology and drama or something equally ridiculous considering the current supply/demand of teaching in this country. The elementary level qualifications are also complicated by prospective teaching students needing only to have a "focus" in a teaching area rather than a full-blown set of studies as with higher division teachers. This is argued as being necessary since the lower division teachers must also fulfill a "breadth requirement" to be able to teach language or history or whatever in addition to the subject(s) they specialize in.
I've found that the best scientific students who express desire to teach are often attracted to the subject and not to teaching itself, and this naturally inclines them toward high school or perhaps junior high school, as they view it as being more "real" or academic. Precious few actual trained scientists (as in people with honours degrees in physics/chem/bio etc) actually want or even consider working with small children, so we are forced to use what we have available, and that is people who do not actually understand the concepts they are required to teach, or else people who have a fluent understanding of high school level science and thus teach it to younger students.
But this is long enough.